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Rh The second mastodon found in 1801 has had a more varied history. In 1803 Rembrandt Peale and his brother Rubens carried it to England. It was exhibited before the Royal Society. While in London, Rembrandt Peale wrote his "Historical Disquisition on the Mammoth." An attempt to sell the skeleton to Napoleon was undertaken, but war broke out with England which prevented the deal being completed. Peak's sons brought it back to this country and they made a southern trip, exhibiting the mastodon as far south as Charleston. Later, in 1813, Rembrandt started a museum in Baltimore on similar lines to that of his father's in Philadelphia. In this museum the mastodon found a home. This became later the Baltimore Museum, from which in 1846 it was purchased by Dr. Warren and taken to Boston for comparison with his very perfect skeleton and placed in the Warren Museum on Beacon Hill. With Warren's mastodon it was bought from the Warren estate by J. P. Morgan and is now in the American Museum of Natural History in New York, and is called the Baltimore mastodon. This is all. that is known of the fate of the collections of Peale's Museum.

A point that has been particularly confusing to historians is the fact that Peale's Museums were located in both New York and Baltimore. In this connection it will become necessary to refer briefly to Peale's sons.

In the last decade of the eighteenth century Rubens Peale and his brother Rembrandt attempted to found in Baltimore a museum with some duplicate specimens given them by their father. This museum was discontinued after one year.

In 1813, however, Rembrandt Peale, who was a better artist than his father, but was less of a naturalist, moved to Baltimore and in the following year opened a museum and art gallery on Holliday Street in a building that afterward became the Baltimore City Hall. The museum finally passed out of his hands, becoming the Baltimore Museum, which was bought by Barnum in 1845. In the early twenties Rembrandt opened a museum in New York, on Broadway, opposite the City Hall. This museum passed out of his hands into that of a stock company, which, after trying to compete with Barnum's American Museum, was obliged to sell out to the great showman in 1842.

When Charles Willson Peale retired, he placed his son Rubens in his place as director of the Philadelphia Museum with the secret hope that with the opportunities at his disposal he would become a naturalist of world-wide reputation. However, Reubens was apparently not much of a naturalist, but during the years that he was director he devoted himself to making the museum self-supporting rather than increasing the value of its collections. For a while, it would seem about 1841, at


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236 the call of the directors of the New York Museum, he became manager of it.

The sons of Peale who had real tastes in zoology were both of the name of Titian. Titian Peale, by Charles Willson's first wife, was becoming a naturalist of great promise and of great help to his father, when he died at the age of eighteen. In memory of this son Peale named a child by his second wife Titian. This Titian became an ornithologist of some distinction, and was conservator of the collections of the Philadelphia Museum for many years.

As we have seen, Charles Willson Peale was an enthusiastic collector. The object of these collections was the education of the public. Peale's ideas as to the function of museums are best illustrated by some extracts from a lecture introductory to a series of forty that he delivered in the winter of 1800-1801. He wrote that a museum should teach the economic use of animals and plants. Says Peale:

A museum should exert a moral influence in the community. Said the lecturer:

After giving a brief account of the history of the museums in the world, from the time of Ptolemy Philadelphus, he describes his ideal museum. Says he:

First let us suppose we have before us a spacious building. . . in which are arranged all the various animals of this vast continent and all other countries. Let us suppose them classically arranged so that the mind may not be confused and distracted in viewing and studying such a vast multitude of objects. Here should be no duplicates and only the varieties of each species, all

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placed in the most conspicuous point of light to be seen to the best advantage, without being handled. Besides a classical catalogue descriptive of every article in so extensive a museum, there ought also to be a library consisting of the writings of the best authors on natural history from Aristotle down to the present time. A few persons well acquainted with the methods of preserving subjects should be continually employed. Gentlemen of talent should be allowed to deliver lectures in the several branches of natural history. . . . It would readily be conceived that some person should have the superintendence of the museum, under whose directions every addition should be made and the care of everything should rest with him. . ..

Parts of this lecture and the one delivered in the previous winter in "the hall of the University of Pennsylvania," as an introduction to a course of twenty-seven, remind one amazingly of certain portions of the addresses of Sir William Flower and Brown Goode.

Some of the characteristic features of Peale's Museum might be summarized as follows:

1. Its collections were educational rather than scientific.

2. The idea of indicating the natural environment with the mounted specimen, the idea of framing the pages of books (recommended also by Brown Goode), the idea of having diagrams and popular descriptions of the specimens beside the specimens themselves, and the idea of placing those of the 4,000 insects that were too small to be easily observed by the naked eye under permanent simple and compound microscopes, and his methods of taxidermy, all of which approach ihe arrangement of modern museums of natural history.

3. Although Peale was not the first to give lectures in natural history in this country, yet he was the first to give lectures illustrated by specimens.

The influence of the museum was wide-spread, but lay not in the direction that the founder had hoped. A perusal of the newspapers of the first decade of the nineteenth century will show that by this time there were a number of museums in every city. They show also that these museums were copied from Peale's Museum, in that they nearly always had a gallery of portraits of heroes in connection with a collection of curiosities. By the first quarter of the century, all had added concerts as an additional attraction. It was not long before the concert developed into a variety theater, although in the case of the Boston Museum legitimate drama was given. In the middle of the century these museums reached their greatest development, such as it was, while Peale's Museum became but a memory. Barnum's American Museum


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238 marked the climax. When in 1902 the Boston Museum ceased to exist, this event marked the end of the last museum that obviously was suggested by the Peale Museum, although there are a few dime museums of later date that have managed to survive.

The cause of the fall of these museums lies in the fact that their place has been filled by natural history museums and zoological gardens which teach true natural history in place of the fake natural history of the dime museum. Although Peale's Museum seems to us to-day a very primitive affair, yet, considering the time when it was founded, the institution must be looked upon in a different light. Then there was no other collection of any kind in the country that could be called a museum. Not having any precedent of museum arrangement, the whole evolution was independent.

The museums of Europe exerted some influence, of course, on its development, but they were so far away that the problems that cropped up from time to time had to be solved independently. This accounts of course, for the many original features that were presented.

The importance of Peale's Museum has been largely discredited, owing to the impression left of its latter days, long after its founder's death. One forgets its positive value and influence when it was directed by his energy and intelligent effort. In the decade of 1800-1810 travelers compare the quality of its collections with the museums on the other side of the ocean. After Peale's retirement from its active direction, the museum ceased practically to grow, while those founded much later began their wonderful development.

Although Peale helped but little to advance our scientific knowledge by the collection of facts, yet to him should be given the credit of organizing what was for awhile a great museum and enabling thousands of people to become acquainted with the appearance and the habits of many animals of this and other countries.


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RGANIC development presents two aspects: that of the individual and that of the race, ontogeny and phylogeny (evolution). These are not two separate and distinct series of phenomena; on the one hand, the individual development is to a certain extent a record of the past history of the race, and the promise of future racial development; on the other hand, evolution is not a series of completed individuals but a series of individual life, histories; for the only road from one generation to the next is by way of a complete life history. Individual development is, therefore, not something distinct from evolution; it is a part of the process of evolution itself; the development of the individual is a chapter in the history of the race.

The development of the individual may be pictured as a steadily broadening stream that takes its source in the fertilized ovum and flows on until death. In this analogy the individual would be represented as a cross-section of the stream at whatever stage we were examining. Though such an analogy limps, inasmuch as individual development is never before us as a unit, as a stream may be conceived to be, and can indeed be said to exist only as the successive cross-sections (its past having disappeared and its future yet unborn), nevertheless, it represents very well the steady, unbroken progress of development from the ovum to old age. There may be crises in the development of the individual, as, for instance, when the chick leaves the egg or the pullet lays its first egg, but there are no breaks in its continuity. Successive generations may be pictured as new streams, each taking its source from a particle—a germ cell—from some cross-section of the preceding generation; and evolution may be represented by placing the new source at a different level than the original. For evolution studies we compare cross-sections of different developmental streams (generations) at comparable distances from the sources, and for evolutionary explanation we must examine the entire series of processes involved in the origin of the new source and in the conditions and inherent character of the new developmental stream.

We can not be said to have actual experience of any other form of development than individual development; evolution or racial


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240 is an inference from innumerable facts and series of phenomena, all of which are bound up together and rendered intelligible by the theory of common descent. We therefore find that the founders of theories of evolution turn to individual development as the court of last. resort, as the place where evolution may be detected in actual process. For here is found the link that binds successive generations, here variations arise, whether they be mutations or of the ordinary fluctuating kind, whether they be germinal or acquired; here in the individual life history the Lamarckian must look for the reflection of the experiences of the individual back upon the germ; here the adherents of orthogenesis must find their crucial evidence.

In his theory of natural selection Darwin accepted as given the data of individual development. But he saw clearly that the fundamental phenomena of heredity and variation had their seat in the individual development, and he experienced the need of framing a conception that would bind together the phenomena of hybridization, the various forms of variation, atavism, telegony, regeneration, inheritance of acquired characters and the like; and in his volumes on "Animals and Plants under Domestication" he framed the provisional hypothesis of pangenesis to include them all. I shall not attempt to present the details of this theory, but I may be permitted to say that, as a matter of logical arrangement of the assumed data, under the circumstances of existing biological conceptions and of the state of knowledge of the time, the theory was well worthy of its illustrious founder. In its way, it was as original as the theory of natural selection, though some of its fundamental ideas had certainly been anticipated by previous writers.

Nor shall I attempt a critical estimate of the value of the theory in the history of science; but I may be permitted to call attention to certain features. In the first place, the theory was overburdened with certain unnecessary conceptions such as inheritance of acquired characters, atavism and telegony. The elimination of these conceptions immensely simplifies the theory of individual development. In the second place, it rested upon a fundamental conception, that of representative particles, which amounts to a denial of the reality of individual development. And in the third place, it assumed certain biological processes—the existence of specific vital particles of ultramicroscopic dimensions, their radiation from parent cells, and their aggregation in other specific cells in a definite architectural pattern—for which there is not only entire absence of evidence, but which are wholly inconsistent with the known facts of cellular physiology. For these reasons the theory had only provisional importance, as indeed Darwin recognized in naming it the provisional hypothesis of pangenesis.

The determinant hypothesis of Weismann, contained in his theory


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Rh of the germ-plasm, includes the assumptions of the pangenesis hypothesis, with those eliminated that were made necessary by the conception of the inheritance of acquired characters. For Weismann's gemmules, or determinants, the assumption of somatic origin was unnecessary, and thus, as Professor Whitman states, the entire centripetal migration of Darwin's theory was eliminated, but the entire centrifugal process was retained. The origin of every character of the individual was explained in the Weismannian theory, as in the Darwinian theory, by the unfolding (it can not be called development) of representative particles. Nevertheless, the theory of the germ-plasm played an important role in the development of biological knowledge, for it framed a set of ideas in a manner sufficiently logical and definite to serve as veritable working hypotheses or bases of attack. The immense effect of Weismann's writings on the theory of individual development should not be underestimated.

The theories of individual development that we have mentioned bear all the marks of provisional or formal hypotheses. Although extremely ingenious and logical, they are based only in small part on analysis of the actual processes and they offer no real explanation of the phenomena themselves; for they really include all the elemental phenomena and merely sum them up; they are definitions that include the matter to be defined; they amount to a denial of the reality of individual development as truly as did the preformation theories of the eighteenth century.

As a series of processes occurring in nature and accessible to experience, the development of the individual is capable of resolution into simpler biological processes, and these presumably into physico-chemical events in the usual sense. All attempts to make such analyses come under the head of Physiology of Development; and this plan of attack on the problems of individual development, known in Germany as developmental mechanics, is one of the most actively pursued lines of biological investigation at the present time. Physiology of Development deals primarily with specific problems, and the results constitute a critical basis for the appreciation of general theories of both individual and racial development. We shall examine some results and principles of these studies, and consider their application to some theories of heredity and evolution.

1. Embryonic Primordia and the Law of Genetic Restriction.—In the course of development the most general features of organization arise first, and those that are successively less general in the order of their specialization. Thus the directions of symmetry of the future organism—the oral and aboral surfaces, right and left sides, anterior


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242 and posterior ends—are the earliest recognizable features of organization of bilateral animals, and they appear while the germ is still unicellular. The distinction between outer, intermediate and internal organs next makes its appearance, each at first as a single tissue. The outer tissue then separates into an epidermal and a nervous tissue, the inner tissue into the intestinal and yolk-sac epithelium, the middle tissue into muscle-forming tissue, connective tissue, skeleton-forming tissue, blood-forming tissue, excretory tissue, peritoneal tissue, etc.

For every structure, therefore, there is a period of emergence from something more general. The earliest discernible germ of any part or organ may be called its primordium. In this sense the ovum is the primordium of the individual, the primitive outer tissue the primordium of all structures of the skin and nervous system, the primitive inner layer of the intestine and all structures connected with it, etc. Primordia are, therefore, of all grades, and each arises from a primordium of a higher grade of generality.

The emergence of a primordium involves a limitation in two directions: (1) it is itself limited in a positive fashion by being restricted to a definite line of differentiation more special than the primordium from which it sprang, and (2) the latter is limited in a negative way by losing the capacity for producing another primordium of exactly the same sort. The advance of differentiation sets a limit, in the manners indicated, to subsequent differentiation, a principle that has been designated by Minot the law of genetic restriction. This in a merely descriptive way is one of the general laws of individual development, and in it is involved the explanation of many important data in the fields of physiology and pathology.

But, though primordia are thus restricted, they nevertheless have the very important property of subdivision, in many cases at least, each part retaining the qualities of the whole. Thus, for instance, in some animals two or several complete embryos may arise from parts of one ovum. Similarly, two or more limbs may be produced in some forms by subdividing a limb bud. Thus frogs with six hind legs have been produced by Gustav Tornier by the simple process of dividing the primordia of the hind legs with a snip of the scissors, in which case he found that on each side one part of the primordium produced a complete pair of legs and the other the normal leg of that side. This capacity for subdivision of primordia explains large classes of pathological facts—at the same time it furnishes a problem to the student of the physiology of development which has proved a serious stumbling block.

2.Principle of Organization.—I have already indicated the existence of direction and localization in the primordial germ of the individual, the unsegmented ovum; the ovum, as we say, is polarized,


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Rh and, not only so, but, in bilateral animals, it is bilaterally symmetrical. This is not usually indicated in the form of the ovum, which is typically spherical, but in the disposition and developmental value of its parts. Here we have one of the most fundamental and least comprehended facts in embryology. It has, moreover, been shown that this property of direction and localization resides in the homogeneous, transparent, semifluid matrix that suspends all the visible particles of the protoplasm of the egg. It is probable that primordia of all grades possess similar properties, and, if this is so, we have a principle that goes far to explain the orderly localization of processes in morphogenesis.

This principle is not farther analyzable at present; but, as it may be found intact in parts of primordia no less than in the whole, it probably rests on a molecular basis. The most ready analogy in simpler phenomena is that of crystallization. The study of fluid crystals has furnished us examples of inorganic molecular aggregates in which direction and localization are given in the whole and also reappear rapidly in the parts when the whole is subdivided.

3. The Rôle of Cell-division in Development.—The individual organism begins as a single cell, from which all cells of the developed organism trace their lineage by the process of cell-division. This has been regarded as one of the most fundamental factors of the individual development in the theories of Weismann, Hertwig and others. But important as the process of cell-division undoubtably is in development, I believe that it is impossible to ascribe to it in principle more than an indirect effect: Considerable complexity of development is possible among Protozoa, whose body is unicellular, and some ova may carry out under experimental conditions a considerable part of the early development without a single cell-division. Moreover, the same kind of differentiated structure may be composed of one cell or of many, or of variable numbers of cells.

The physiological value of cell-division is no different in principle in developing than in functioning tissues (using these terms in the usual sense). The general law of relative reduction of surface in proportion to increasing mass imposes a size limit on cells, which can be regulated only by cell-division; an internal principle of regulation of cell-size has also been stated by R. Hertwig and Boveri, viz., a certain relationship characteristic of each species between the amounts of nuclear and cytoplasmic matters, so that increase of initial volume of the former involves increase of the latter, and vice versa. Corresponding to these principles, we find that individuals of different sizes of the same species vary not in the size, but in the number of the cells; and this is regulated by variation in the number of cell-divisions in different individuals.

Cell-division must necessarily, therefore, have an immense {{hws|func|functional}


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244 significance in development, owing to the principle of relation of functional area to mass. It has also another very important function as an isolating factor. The localizations that arise, owing to the organization process of which we have spoken, are rendered relatively stable and permanent by the formation of cell-walls. Thus the elements of the mosaic are isolated, and each isolated part has the opportunity to grow into a new mosaic. Cell-division is thus an important factor in progressive differentiation, not as a cause, but as a means.

4. Environment.—Environment must be conceived in a somewhat broader sense than usual in considering the individual development. The developing embryo has an environment in the usual sense, consisting of all those external conditions that surround it, some of which enter into its development. But in addition to this extra-organic environment there is an intra-organic one; the developing embryo is not merely a unit on which an extra-organic environment operates, but it is a living mosaic, each element of which may conceivably enter into the development of any other in the sense of being a factor in the process. Each part of the embryo, therefore, has an intra-organic environment consisting of all the other parts, 6ome of which constitute relatively immediate environmental factors, others relatively remote ones.

To illustrate: nerves arise in the embryo from certain centers and grow out in the embryonic tissues, much as roots grow out in the soil; the muscles arise separately and the nerves grow to them and make the proper connections. Is this due to an innate tendency of each nerve to grow in particular paths and branch according to definite laws, or, on the other hand, is it due to a directive stimulus exerted on the growing nerve by developing muscle tissue? The answer can be given only by a suitable experiment: If an abnormal innervation area were brought into the field of growth of a developing nerve, would the nerve entering the abnormal area follow its normal mode of branching, or the one characteristic of the normal nerves of the transposed area? To be specific: the bud of a leg of a tadpole that has as yet no nerves may be transplanted to any region of the body (Braus and Harrison), and it develops as a leg; but it receives its innervation from the nerves of the region to which it has been transplanted, and the mode of branching of the nerve is that of the leg nerves. We may generalize this statement by saying that any nerve may be made to depart from its normal mode of branching and to branch like leg nerves, by bringing a leg bud into its innervation area at the time that the nerve is still growing.

It will be seen that if this is generally true, the constancy of distribution of peripheral nerves is not due to the transmission of nerve-branching determinants from generation to generation, but is a function of the intra-organic environment in each generation.


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Rh The case of the determination of nerve-branching by intra-organic relations does not by any means stand alone. The same principle undoubtedly holds for the development of the blood-vessels, which grow along paths determined by the arrangement of organs and tissues and not according to a predetermined law given in the blood-vessels themselves. Color patterns have been shown in some cases to be determined by intra-organic variable relations, as in Loeb's experiments on the determination of the color pattern of the yolk-sac of a fish, which he demonstrated to be due to the positive attraction of the circulating blood for migratory cells that bear pigment. The development of any color pattern was therefore dependent upon blood-circulation, and the form of the pattern upon the pattern of the blood-vessels. The primordia of the eye or the ear transplanted to strange locations in the embryo induce formations in surrounding tissues that are strange to them and characteristic of the normal eye and ear environment. The origin and growth of motor nerve cells has been shown in my laboratory by Miss Shorey to be dependent in the chick on normal muscle development; so that the anatomy of the central nervous system, no less than the peripheral system, is dependent to some extent on the environment. Regeneration of lost parts is dependent for its completion to some degree on innervation, and the normal development of muscle tissue beyond a certain stage is likewise so dependent. These examples might be increased by others, which, taken together, would show that an immense part of what we call inheritance is inheritance of environment only, that is, repetition of similar developmental processes under similar conditions. The bearing of all this on the doctrine of determinants, that characters of the adult are represented by germs of a lesser order in the germ of the entire organism, is obvious.

Many of the problems of heredity, so-called, are not capable at present of such resolution. We may note some instances of this kind and then attempt to analyze the whole matter briefly. The example cited of transplantation of a leg-bud is of this kind: the transplanted leg-bud does not develop into an arm if it be transplanted to the region of the arm, but into a right or left leg, as the case may be, and this is true no matter how early the stage at which the transplantation may be made. It is not possible to change the specificity of such a primordium by any means yet employed. Moreover, there are many other experiments which show that the primordia of a great many structures are definitely specified even before they can be detected by any method of pure observation. Thus if a portion of the medullary plate of a frog embryo be cut out so as to include in the cut part the region that would form an eye in the course of time, and if then this piece be replaced inverted, it is found that the subsequent development of this area is inverted, not restored to the normal, although no trace of organs


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246 was present at the time of the operation (Spemann). In this case, then, the eye appears in an abnormal position.

Correlative Differentiation.—We have cited a series of cases that illustrate two apparently contradictory principles known as the principles of correlative differentiation and of self-differentiation. The part that these play in embryonic development should be analyzed. The data of correlative differentiation may be placed in two categories, one of behavior and one of metabolic relations. Considering these separately:

Behavior.—Any case of behavior involves a stimulus, and a response; these imply irritability and reaction capacity. To take a simple case, for instance, the contraction of a muscle, the stimulus may be of a variety of kinds, nervous, chemical, electrical, thermic, mechanical; in any case the response is contraction. The nature of the response is given in the system and is limited by its reaction capacity. The muscle cell does not contract for one kind of stimulus and secrete in Response to another.

This principle is elementary in physiology and psychology and it must apply also in the physiology of development. It appears to me that it has not been sufficiently borne in mind by students of the subject. Herbst, for instance, divides developmental stimuli into directive, trophic and formative. The first kind of stimulus determines the direction of growth or migration, and so plays an important part in development, a really great part illustrated in two of the cases cited, viz., the mode of branching of nerves, and the direction of migration of wandering cells. Trophic stimuli are those that affect the rate or :amount of growth without altering its specific character.

The conception of formative stimuli implies, if it has any meaning whatever, that the nature of a developmental process is determined %y the nature of a stimulus. A case often cited is as follows: the two most fundamental parts of the eye, lens and retina, develop from two entirely distinct primordia, the retina from the embryonic brain and the lens from the epidermis. The retina first grows out from the wall of the brain and reaches the epidermis to which it becomes fused. The latter then produces a lens. Now it was shown for some amphibia, that, if the retina fails to reach the epidermis, no lens forms; therefore, it was argued that the production of the lens is due to a formative stimulus exercised by the retina on the epidermis. But in some other cases the lens forms even if the retina be absent; which does not prove that it arises without stimulus, only that this specific stimulus is not needed. And the fact that transplanted optic vesicles stimulate lens formation in strange localities from the epidermis merely shows that this form of reaction of embryonic epidermis is widespread at this stage of development.

The instance is valuable as proving that stimuli are important in


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Rh development, but useless as an example of a formative stimulus. Morphogenetic behavior, like behavior in other fields, is not a function of the stimulus as to its specificity, but it is prescribed and limited by the reaction capacity of the system. One example is as good as many. We shall not find this principle contradicted by any of the known data of the physiology of development.

Metabolic Relations.—When we consider to what an extent the nature of every biological character is given in its chemical composition, it can be readily understood that, to some authors, physiological chemistry should seem the complete basis of heredity. The characters of every tissue of the body are absolutely dependent on their chemical composition, and even slight variations in chemical composition may completely alter function, appearance or form. For such a statement examples are entirely unnecessary.

The development of characters in the individual is dependent upon the occurrence of definite chemical reactions, upon their rate and upon their degree of completion. It has been shown that the law of acceleration of embryonic development in correspondence with rise of temperature is the same in principle as the law of acceleration of chemical reactions by temperature increase. Numerous experiments have been made on the character of development in the absence of one or other or combinations of the elements normal to protoplasm, with the aim of determining their rôle in development. Herbst, for instance, has made a series of experiments on the development of larvae of the sea urchin in artificial sea waters, in the composition of which definite elements are wanting. He shows, for instance, that in sea water made up without calcium the skeleton fails to develop, and that the form of the larva resulting is profoundly modified from the normal. In other experiments the potassium or sulphur, or iron, etc., is omitted from the solution, and the effect on the development noted. Other experimenters have maintained that the presence of specific chemical elements in excess has definite morphological consequences.

As regards complex substances and their rôle in morphogenesis, but little is actually known. Recent results indicate that the egg contains substances of complex chemical composition which are essential for the development of specific parts or tissues. Thus certain experiments consist in the removal of definite parts of the egg containing specific materials; and in the subsequent development specific parts of the embryo are wanting. In other experiments, by Conklin, the transference of definite substances from their normal location by means of centrifugal force is followed by the development of corresponding


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248 specific structures in the abnormal location. These experiments strongly suggest, even if they do not rigorously prove, that such substances are essential ingredients in definite developmental processes. I am indebted to Dr. Riddle for the following illustration: The various colors of mammals, such as black, brown, red, yellow, are due to chemical substances known as melanins. The chemistry of these substances starts out from a simple colorless base or chromogen, from which the series of colors, yellow, red, brown, black, is derived as successive stages of oxidation. The chromogen base is found in all mammals; the color then would appear to be due to the varying powers of the cells of different individuals to oxidize the given base. Tornier has shown in his experiments on the coloration of Amphibia that the particular color developed is a function of nutrition, varying in the order of oxidation value (as was later ascertained) according to the degree of nutrition. The development or inheritance of color, therefore, can certainly not be due to the presence of black or brown or red or yellow determinants in the germ, assumed for theoretical purposes by some students of heredity, but to a specific power of oxidation of the protoplasm. This faculty in its turn is no doubt capable of resolution into other physiological terms.

We are only at the beginning of the study of correlations of embryonic metabolism. The role that the internal secretions of the embryo may play in the processes of development is practically unknown; but we may expect to find here biological reactions of fundamental significance, especially when we consider such phenomena of the adult as the influence of pregnancy on the organism, the possibility of inducing lactation, with all that this implies, by injection of fœtal tissues; the relations between the sex organs and secondary sexual characteristics and indeed the entire habitus of the organism; the influence of a small gland like the thyroid, or the pituitary body, etc. Biochemical reaction runs through every phase of development and is unquestionably the decisive factor in the appearance of many characters of the organism.

Self-differentiation.—The conception of self-differentiation in morphogenesis is a vague and unsatisfactory one. In a sense it is a contradiction in biological terms, for assuredly environment enters into every biological process. On the one hand, the term covers the fact of the specificity of primordia, which means only a certain stability of metabolism and reaction capacity; on the other hand, it may have specific meaning in one large class of developmental phenomena, viz., polarization and localization. If, for instance, the term self-differentiation might be applied to the appearance of definite axes, angles, points and faces of a crystal, it would with equal propriety be applicable to the appearance of polarity, bilaterality, etc., the axes of embryonic development. But if the term should come to hold simply this restricted meaning, then all reason for its maintenance would be gone.


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Rh It is not at all certain that it will be possible to reduce all the problems of the physiology of development to such categories as we have mentioned. The subject is full of unsolved problems, but so far as I can see no one has shown any real reason for assuming ultraphysical agencies in any of the events, and there is the same pragmatic reason for refusing to assent to such suggestions, which are made all too frequently, that there is in other fields of science. If we will be consistent, we are driven to the conclusion that the apparent simplicity of the germ is real, that the germ contains no gemmules, or determinants or other representative particles; that development is truly epigenetic, a natural series of events that succeed one another according to physicochemical and physiological laws; the explanation of the sequence consists simply in the discovery of each of its steps.

The problems of heredity and variation are included in a true physiological conception of the individual development; but some biological conceptions that have more or less status and reputation are inconsistent with it. Such are the inheritance of acquired characters, atavism, and the theory of unit characters. The first is a familiar problem that I shall not argue anew; the second logically implies the presence of ancestral representative particles in the germ, which is inconsistent with a physiological theory of development. But it is obvious that the facts united under the name of atavism or reversion take their place naturally in a physiological theory of development, as arrests of development, or modification of environment, or in other ways.

The theory of unit characters deserves more attention for it ia essentially a modern theory, and counts numerous adherents. This, conception has been most sharply formulated by De Vries in his Mutationstheorie. He says:

Bateson's allelomorphs constitute a similar conception. Such hypothetical elements of organization must be conceived as distinct from the germ on. They can be shuffled about from one generation to another, and can, therefore, be introduced, removed or replaced in the germ cells. It must be admitted that these conceptions fit certain facts of inheritance in many hybrids fairly well, but the progress of discovery has made necessary the installation of subsidiary hypotheses, so that the most recent conceptions of unit characters are becoming extremely


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250 complex, and it would seem as though the system would soon fall of its own weight. The entire value of the hypothesis consists in the formal approximate expression of certain facts; when it is found that the hypothesis begins to fail even for the classes of facts for which it was originally intended, and that most of the known facts of development can not possibly be expressed in its terms, the entire conception is put on trial.

The weakness in the theory of unit characters is in the use and conception of the term "character.' The term has been prescribed to us by the systematic zoologists and botanists engaged in describing the differences between species; so that "character" really means any definable feature of an anatomical kind that differentiates species; by extension it also means any other differentiating features that can be defined. In the study of evolution and heredity, it is usually only anatomical characters that are in question. Now the study of the physiology of development teaches us that whatever else "characters" may be, they are not units; they simply represent the sum of all physiological processes coming to expression in definable areas or ways, and they may thus represent a particular stage of a chemical process, or a mode of reaction of some part. "Character" is essentially a static morphological term; in the study of heredity and development we are dealing with biological processes. To adapt a phrase of Huxley's: "characters" are like shells cast up on the beach by the ebb and flow of the vital tides; they have a more or less adventitious quality. To give them representation in the germ is equivalent to a denial of uniformity in biological phenomena.

Just as the exact position of each shell on a beach might be fully explained if we knew its full history, so each character has a certain kind of inner necessity as the result of a sequence of developmental processes. And just as in the history of the position of the shell on the beach we should certainly ascribe great importance to the tides and winds, so in the quality of each individual character we should find corresponding vital tides and winds, as regular and lawful as those of the ocean. We do not yet know the secrets of the vital tides; we maintain only that they are the moving forces in development and heredity, just as in physiology and pathology; and every fundamental contribution to the physiology of protoplasm is at the same time, and to the same extent, a contribution to heredity and the physiology of development.

But if these principles are accepted, how are we to explain the facts on which the theory of unit characters depends? The main difficulty lies not in the facts of mutation, for the physiology of this phenomenon already begins to appear from the experiments of Tower and MacDougal, who show that mutations may result from action of


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Rh environment directly on the germ cells. The most fundamental phenomena in the unit character theory are unquestionably the segregations of characters that appear in the offspring of hybrids in so-called Mendelian inheritance. In the most typical cases, grandparental characters reappear in definite proportions of the progeny of the hybrid generation. The interpretation, according to the theory of unit characters, is in the hypothesis of purity of the germ cells of the hybrid generation with respect to the segregated characters; which means that the germ cells of the hybrid generation are pure with reference to the contrasting characters united in the soma; in other words, that corresponding contrasted characters can not both remain in the same germ cell, but are segregated in different ones and may thus appear pure in the descendants of a hybrid generation.

We may well doubt that absolute purity of grandparental characters in the offspring of the hybrid generation occurs, and the results unquestionably vary with the environment; but I believe that we have to admit the general principle of segregation. However, the theory of segregation of unit characters in the germ cells is in no way necessary to explain the results; it is in fact inconsistent with the highly variable result; if unit characters were segregated in the germ, we should expect very definite constant results.

If we take our stand on the epigenetic basis and regard the germ cells as no more complex than direct investigation would lead us to suppose, then we have to admit that segregation in the germ cells can involve only constituents of the germ cells themselves. But any variation thus induced in the germ cells would be a factor in each process of the development, and would hence tend to influence every character that appears. Such a hypothesis involves the conception that germ cells contain elements capable of segregation; and this is so. Even if the principle of segregation of characters in inheritance had never been discovered, the principle of segregation of germ-cell elements would still hold, for the two discoveries were made absolutely independently.

I refer to the work of Guyer and Montgomery on the chromosomes, which has been followed by a long series of very exact studies. These studies certainly suggest segregation of parental chromosomes in varying proportions in different germ cells. Indeed, I know of no other interpretation of chromosome behavior that is consistent with the facts. Whatever value we may attribute to the chromosomes in cellular physiology, the variable relations established by their differential segregations, even if only quantitative differences are concerned, must involve endless secondary effects in the long series of cell generations that make


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252 up the individual life history. It is not impossible that other segregations than those of the chromosomes form part of the germ-cell behavior, but of this we know nothing as yet. In any event, the principle of segregation of actual visible elements of the germ cells has a firm anatomical basis.

It must not be forgotten that the germ is the entire organism and that it passes through development as the same individual; continuity of individuality is preserved throughout development. Therefore, if we discard determinant hypotheses and take our stand on a strictly physiological theory of development, it follows of necessity that the transmitted factors of heredity included in the organization of the germ cells must be factors in the development of the entire organism. The so-called Mendelian factors must therefore be of this character, as I have argued elsewhere. That is to say, the segregated factors must be general constitutional conditions effective as factors in the development of every part of the organism. It can readily be seen that specific intensity of metabolism, or of reactivity, and variation in constitutional size of cells may be such conditions. Others no doubt exist, of which sex may be one. The essential thing to recognize is that the heritable and segregable factors, being conditions of the germ cells at the start, can never be anything less than factors of the entire organism at all stages.

Our conclusion is that the theory of individual development must more and more come to be regarded as a branch of physiology proper. The theory of representative particles must be relegated to the class of formal hypotheses whose usefulness is largely outlived. While it may still play a part in speculations on heredity, I believe that it will come to be generally recognized by those who use it as a mere matter of convenience of terminology, and not as an explanation of the phenomena described in its terms, in the sense of being a verifiable part of the sequence of processes in development.


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N dealing with the differentiation of nervous organs, the earthworm affords a good example of a simple type of well-centralized nervous system. The central nervous organs in this animal (Fig. 1) consist of a brain or cerebral ganglion situated anteriorly and dorsal to the buccal cavity, right and left œsophageal connectives extending from the brain ventrally to the ventral nerve-cord which stretches as a segmented organ from near the anterior end of the worm over its ventral line posteriorly to the tail. The segments in the ventral cord agree in

number and position with those of the worm's body and from each segment three pairs of nerves pass out to the integument and muscles of the adjacent region.

The essential nervous elements of the ventral cord can be made out in transverse sections (Fig. 2). In such sections the integument will be seen to be filled with sense-cells, each of which ends peripherally in a sensory bristle and gives rise centrally, in addition to a few sub-epithelial processes, to a single nerve-fiber which passes inward between the muscles and enters the ventral ganglion by one of its three nerves; finally this fiber spreads out in the fibrillar substance or neuropile of the ganglion. This cell-body in the integument with its processes including the nerve-fiber constitutes a primary sensory neurone. These neurones usually do not spread beyond the ganglion with which they are directly connected, but in exceptional cases they may extend into the ganglion anterior or posterior to this one.

In the ventral and lateral portions of each ganglion are numerous large nerve-cells from which coarse processes extend through the


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254 pile, fibrillating as they pass to terminate as motor nerve-fibers in the muscles of the adjacent part of the body. These cells with their processes constitute the primary motor neurones of the earthworm and, like the sensory neurones, they may be present in any one of the three nerves of a segment. Their longitudinal extent is probably not much beyond a single segment.

The primary sensory and motor neurones not only give rise to the nerves of the earthworm, but they contribute a larger part of the substance

of each ganglion. As stated in the first article, they form when together the necessary elements for the simplest, conventional reflex-arc. How they are related to one another in the neuropile is not conclusively settled, but, judging from the work of Apáthy (1897) and others, the connection here as in the nervous net is one of direct continuity.

Besides the motor and sensory neurones, the central nervous organs of the earthworm contain a considerable number of so-called association neurones. These are nerve-cells with longer or shorter processes that connect parts within the same ganglion or run from one ganglion to another. They give rise to no fibers that extend into the nerves and hence they are strictly limited to the central nervous organs. Their longitudinal extent is seldom over more than one or two segments.

Since the sensory, motor, and association neurones thus far described make up the bulk of the ventral nerve-cord of the earthworm and since none of these have a longitudinal extent of more than a few segments, it follows that the cord must be conceived as made up of an immense number of overlapping short neurones which in this collective way stretch over its hundred and twenty or more segments. But the nerve cord of the earthworm is not composed exclusively of short neurones. In its dorsal portion are three giant fibers which, though their nature has been even recently disputed, are without much doubt nervous


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Rh organs. The middle and largest of these fibers extends almost the whole length of the ventral cord and, according to Friedländer (1894), has unquestionable connections with ganglion-cells. The two lateral fibers, though smaller, have much the same extent as the median one and are also directly connected with cells. Both sets of fibers connect by branches with the neuropile of the successive segments. Thus the ventral cord of the earthworm may be described as composed of three long neurones and an immense number of overlapping short neurones.

This peculiarity in the structure of the cord makes itself manifest in the movements of the worm. Undoubtedly the slow waves of muscular activity that move over the worm from head to tail as it creeps along are dependent upon the interlocked short neurones, whereas the

 Segalion (modified from Hatschek). bm, basement membrane; c, cuticula; e, epidermis; gc, ganglion-cells; n, nerve-fibers and neuropile; s, space occupied by vacuolated supporting tissue.

sudden drawing together of the worm as a whole, when it is vigorously stimulated, is very probably the result of impulses spread through the long neurones.

The absence of degenerated fiber-tracts in the ventral cords of earthworms that have been cut in two and the rapidity with which nervous regeneration takes place in these worms are conditions that very likely depend upon the almost entire formation of the cord from systems of short neurones.

At first sight the central nervous apparatus of the earthworm seems to be widely different from the neuromuscular mechanism of the cœlenterates, but the difference in reality is not so pronounced. To begin with, the whole nervous mechanism of the cœlenterate is within an


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256 epithelial layer, whereas the central nervous organs of the earthworm are solid masses of nerve-cells, fibers, and neuropile entirely distinct from any epithelium. But this condition is apparently a recent acquisition on the part of the earthworm, for in another annelid, Sagalion, the ventral cord (Fig. 3) and the brain are still a part of the superficial

 showing the ganglion-cells on the ventral side (v) and the nerve-fibers and neuropile on the dorsal side (d).

ectoderm and differ from the condition in the cœlenterates only in that they represent a concentration of nervous elements in certain regions instead of a diffuse condition as in the sea-anemones, etc. In Nereis the brain is epithelial, but the cord by a process of delamination has broken away from the integument, and in the earthworm the whole central nervous system, brain as well as cord, has delaminated. It is chiefly this concentration and separation of the nervous organs from the skin that justifies, in my opinion, the statement that an earthworm has central nervous organs and a sea-anemone has not.

The fact, however, that the central nervous system of the earthworm has developed on the lines of the cœlenterate, has left its mark in the distribution of nervous materials in the ventral cord of this animal. In the ectoderm of the cœlenterate the cell-bodies of the nervous mechanism are nearer the exterior of the animal than are their processes, the fibrillar mass, and the same is true in the ventral ganglia of the earthworm (Fig. 4); here the cell-bodies are on the ventral side of the ganglion, i. e., next the integumentary epithelium, and the neuropile and nerve-fibers are on the opposite or dorsal side of the ganglion. This peculiarity in the distribution of nervous materials is apparently true for most higher metazoans.

Another point of comparison between the nervous mechanism in cœlenterates and in the earthworm is the presence of nerves in the latter and their absence in the former. As already pointed out, the nerves in the earthworm are bundles of independent fibers which course more or less together between their end-organs and the central apparatus.


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Rh The fibers in a nerve have no necessary functional relations one with another, but are brought together chiefly by convenience of passage. They are characteristic of those animals in which sense organs and muscles have become well differentiated and widely separated from the central organs, and are not to be confused with elongated bundles of nervous elements such as are to be met with in some cœlenterates and many echinoderms, for though these may represent early steps in the evolution of nerves, they still retain so many evidences of functional interrelation among their elements that they are to be classed rather with nervous nets than with nerves.

The differentiation of nerves as thus defined implies an increased interrelation of neurones in the central apparatus as compared with the condition in the more primitive nervous net. The nature of this growing interrelation has been well expressed by Sherrington (1906) in his principle of the common path. This principle implies that each sense organ may be connected through the central organ with every effector and conversely any effector may receive through the central organ impulses from any sense-organ. In consequence the central organ must contain many common paths which are momentarily used, now for this, now for that combination of particular receptors and effectors. This condition without doubt obtains in earthworms as it does in higher animals, and is a feature that can hardly be said to exist in the nervous nets of the cœlenterates.

It is also probable that the nervous mechanism in cœlenterates differs from that in the earthworm in its capacity as a nervous transmitter. Attention has already been called to the fact that transmission in the nervous net of a cœlenterate may occur in almost any direction and that in the central nervous organs of vertebrates it is very definitely limited and may in fact flow in only one of two apparently possible directions. So definite a restriction can not be asserted for the earthworm but, as Norman (1900) has shown, significant differences do obtain. If an earthworm that is creeping forward over a smooth surface is suddenly cut in two near the middle, the anterior portion will move onward without much disturbance whereas the posterior part will wriggle as though in convulsions. This reaction, which can be repeatedly obtained on even fragments of worms, shows that a single cut involves a stimulation which in a posterior direction gives rise to a wholly different form of response to what it does anteriorly; in other words, transmission in the nerve-cord of the worm is specialized as compared with transmission in the nervous net of the cœlenterate.

There is good reason to believe that the cerebral ganglion or brain of the earthworm is in a measure degenerate. Certainly if we turn to such an annelid as Nereis we find in place of the small mass of ganglionic cells and fibers that represent the brain in the earthworm a much


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258 more extensive organ connected with a considerable number of sense organs none of which are present in the earthworm. Eight peristomial tentacles, a pair of palps, a pair of antennæ and, two pairs of eyes are found connected by nerves with the brain of Nereis and represent a condition in strong contrast with the unspecialized state in the earthworm. Yet both the earthworm and Nereis show much the same traits when deprived of their brains (Loeb, 1894). Each worm is immensely reduced in activity somewhat as a jellyfish is after the removal of its sense-bodies, and one is justified in concluding that the head of even the earthworm is an especially sensitive region through which many slight environmental influences that might not be able to affect other parts of the body gain access at this point to the neuromuscular mechanism. That such a condition should obtain at the anterior end of a bilateral animal has long been recognized as appropriate, for this is the part of the animal that in normal locomotion first reaches the new environment. But I am not acquainted with any discussion as to the mutual relations of the nervous parts at the anterior end of an animal so far as their origins are concerned. If what has been said in these lectures is true, namely, that sense-organs in general precede central nervous organs in evolution, then the brain of the worm has developed at its anterior end because the chief sense-organs were originally there, and not vice versa, a statement that I believe to hold for the growth of the brain in all animals. Intricate and marvelous as the brain of the higher animals is, it is, in my opinion, the product of a group of sense-organs that in evolution preceded it in point of time.

The annelids then possess a neuromuscular mechanism in which there are not only primary organs such as muscles, and secondary organs, the sense-organs, but also tertiary organs, central nervous organs. These central organs intervene in position between the receptors and effectors and in the annelids are composed almost exclusively of short overlapping neurones. It is probable that in the sea-anemone these neurones are represented by the so-called ganglion-cells of the nervous layer, but I would not go as far as Havet (1901) and designate these cells in cœlenterates as motor cells, for though some of them undoubtedly connect with the muscle-fibers, others may be purely association neurones. I believe further that in the sea-anemones the fibrils from many sense-cells connect directly with muscle-fibers without the intervention of ganglion-cells.

As an example of a central nervous system built upon the annelid type but with increased complication, we may turn to the arthropods. The central nervous system of these animals, like that of the annelids, consists of a dorsal brain, œsophageal connectives, and a ventral, segmented cord. These organs have been formed by a process of delamination as in the earthworm and exhibit the same fundamental


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228

hard work they were rewarded with grand success and were able to ship to Philadelphia one skeleton that lacked principally the lower jaw and the top of the head. What was lacking in this skeleton was found in another from a nearby bog. The bones of the two animals were not mixed. When the skeletons were set up the missing parts were carved out of wood, so that there were finished two complete skeletons. "Although putting these skeletons together" to return again to the autobiography, "was a long and arduous work; yet the novelty of the subject, the producing the form, and, as it would seem a second creation, was delightful; and every day's work brought forth its pleasure."

Up to this time many scattered bones and teeth of the mastodon had been found in this country. They had been described as belonging to a race of gigantic man, to the fathers of cattle, to hippopotomi, etc. In a letter to Geffroy Saint-Hilaire describing his find Peale states that this animal should be called the carnivorous elephant of the north, but should not be confounded with the Siberian mammoth. Cuvier in his memoir "Le Grand Mastodonte" writes:

Mais pendant que nous travail lions ainsi en Europe sur quelques fragmens de cet animal, M. Peale continuait à en recueillir les os, et il avait été assez heureux pour en obtenir deux squelettes presque complets qui ont décidé la question pour toujours (p. 261).

In 1802 the state legislature moved to Lancaster. This left the State House (Independence Hall) vacant. Peale petitioned the


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Rh of cellular elements as is seen in this animal, i. e., the ganglion-cells are on the side of the cord next the exterior, and the neuropile and nerve-fibers next the interior.

The chief fundamental point of difference in the nervous systems of the annelids and arthropods consists in the great number of long neurones in the latter as compared with the former. In the crab, as demonstrated by Bethe (1897), many of the primary sensory neurones extend over half the length of the ventral cord instead of being limited to a few segments as in the earthworm, and the same is true of the primary motor neurones. Moreover, the association neurones have shown an extensive growth. Although in the crab there are some neurones limited to one or two segments, as is the rule in the earthworm, the great majority extend over many segments and often through the whole length of the nervous system. In this way the central nervous organs of these animals are locked together much more closely than are those in the worm and exhibit consequently in their physiology a unity that the worms do not possess. This nervous unity, moreover, has developed to such a degree in the higher arthropods that we may with reason ascribe to such animals as the insects a primitive form of intellectual life not unlike that found in the vertebrates. The structural basis for this seems to me to be foreshadowed in the few long neurones of the worm which, as I have just pointed out, come to be the common type in the arthropods. The type of central nervous system with long neurones also characterizes the other higher invertebrates such as the mollusks, etc.

The central nervous system of the vertebrates and of certain other closely allied forms like the tunicates, is usually put in strong contrast with that of the higher invertebrates. The most striking feature in this contrast is the fact that the vertebrate nervous system is tubular and the invertebrate solid. As is well known, the central nervous organs in vertebrates develop from an ectodermic tube that has been infolded from the median dorsal surface of the animal. This simple nerve-tube with nervous connections, but otherwise almost unmodified, exists to-day in that primitive vertebrate amphioxus. In the higher vertebrates the posterior portion of this tube becomes uniformly thickened and forms the spinal cord, the central canal of which gives evidence of its tubular nature. The anterior portion undergoes still more profound changes than the posterior part in that its wall thickens very differently in different regions and expands in several lobe-like outgrowths, giving rise thus to the brain whose ventricles represent the original cavity of the nerve-tube.

Notwithstanding the striking difference between the central nervous organs of vertebrates and invertebrates, they show certain fundamental similarities and the first of these has to do with the distribution of


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260 nervous materials. Since the nerve-tube from which the central nervous organs in vertebrates are developed is infolded ectoderm, it follows that the inner surface of the tube represents a portion of the outer surface of the animal. This inner surface even in the adult central nervous system is always covered by an epithelium as the exterior of the animal is, and the nervous materials which surround it are related to this epithelium in a characteristic way. This relation can be most easily seen in any transverse section of the spinal cord. Beginning at the central canal of such a section (Fig. 5) and proceeding

 c, central canal; e, epidermis; g, gray substance composed of ganglion cells and neuropile; w, white substance or nerve-fibers.

through the substance of the cord to the opposite face, one passes first an epithelial layer, then gray substances composed of nerve-cells, neuropile, etc., and finally white substance made up of nerve-fibers. Precisely this sequence is met with in the central nervous system of any primitive invertebrate such as Segalion, where, as already pointed out, in passing through the thickness of the central nervous organ from the exterior to the interior one meets first external epithelium, then ganglion-cells and fibrillæ corresponding to the gray substance of vertebrates, and finally nerve-fibers corresponding to the white substance of these animals. Thus the nervous materials of the vertebrate spinal cord are distributed through that structure on a plan similar to that found in invertebrates, and this plan, though considerably modified, also holds good for the vertebrate brain. So far as these particulars are concerned, the vertebrate central nervous system differs from that of the higher invertebrates chiefly in that in separating from the integument it has carried with it its epithelial mother-tissue instead of leaving this tissue behind.

Not only are the materials of the vertebrate central organs distributed on a plan that is best understood from the standpoint of the invertebrates, but the primary neurones of vertebrates are also most clearly interpreted from this point of view. The primary motor neurones of


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Rh vertebrates (Fig. 6) resemble very closely those of invertebrates, for their cell-bodies are within the central nervous mass and their neurites extend as motor nerve-fibers to the skeletal muscles. The primary sensory neurones also agree with those of the invertebrates except that their cell-bodies instead of being in or near the integument, as in most invertebrates, have migrated centrally and thus form the dorsal ganglia. At least this appears to have occurred in all vertebrate sensory nerves except the olfactory, which still retains the usual invertebrate condition.

 c, spinal cord; dg, dorsal ganglion; i, Integument; m, muscle; mn, motor neurone; sn, sensory neurone.

Association neurones, which were met with in the invertebrates, are abundantly present in the vertebrates.

How the neurones in vertebrates are related to one another has been a matter of much dispute. Whether the gray substance of the central organs in these animals contains a true nervous net as seems to be the case in many invertebrates or whether their neurones retain greater individuality and are related morphologically only through contact, is not yet settled. That many embryonic neurones, or neurocytes, are in the beginning widely separated from others with which they are ultimately closely related is true and gives color to the belief that they may never fuse anatomically, though physiologically they do become continuous. The fact that nervous transmission through central organs in adult vertebrates is slow, open to exhaustion, and restricted to one direction as contrasted with transmission through nerve-fibers, is strong physiological evidence of a special central mechanism of interrelation between neurones such as Sherrington (1906) has pictured in the synapse. That no special anatomical condition has thus far been discovered that answers to this physiological requirement can in no sense be taken as an objection to it. That the vertebrate central nervous system is in many of its parts a synaptic organ can not be doubted, but that all its parts are synaptic is not yet proved. Possibly this is a


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262 feature characteristic of only the more specialized parts of the vertebrate central organs and entirely absent from the invertebrate, but whether this difference really exists or not must remain for future investigation.

Although it can not be said at present that a synaptic nervous system is the peculiar possession of the vertebrates, there are two important features in which the central organs of these animals differ from those of the invertebrates. In the first place, the central organs of vertebrates exhibit a large prepondernacepreponderance [sic] of long neurones over short ones, and in the second place, they show an enormous increase in the number of association neurones. In an earthworm there are only three long neurones and the rest are short ones; in a crab the long and short neurones are perhaps about equally abundant; but in a vertebrate the long neurones certainly far outnumber the short ones. In any transverse section of the spinal cord of one of the higher animals almost all of the white substance in view excepting a thin layer surrounding the ventral horn is made up of systems of long neurones. In this respect the condition in the vertebrates seems to be almost the reverse of that in worms and in consequence transection of their central nervous organs results in profound and extensive degeneration such as is never met with in animals like worms. For this reason the central nervous system of the vertebrate, though giving much evidence of segmentation in its early stages of growth, is finally a physiological unit such as is realized in no other group of animals, a condition well evidenced by the fact that some of its most recent phylogenetic acquisitions, like the pyramidal tracts of the mammals, may consist of neurones that reach almost from one end of the system to the other.

The second feature that distinguishes the central nervous organs of vertebrates from those of invertebrates is the enormous development of association neurones. These neurones are present in worms, are numerous in arthropods, but are overwhelmingly abundant in vertebrates. Of the white substance seen in the transverse section of the spinal cord almost all except the dorsal columns represent association neurones. Judged from this standpoint there are certainly many more association neurones in the cord than all other kinds taken together. But the association neurones are not only the most numerous in the vertebrates; they also constitute the basis of the most significant evolution. The central nervous organs that show the most conspicuous progressive changes in the vertebrates are the cerebellum and the cerebrum, particularly their cortical portions, and when it is remembered that few or no primary sensory or motor neurones contribute to these two organs, but that they are made up of association neurones almost exclusively, it will be seen how enormously important these neurones become. The association neurones in the vertebrates are not only the organs of intricate nervous exchange, but in the region of the cerebral cortex they


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Rh afford the material basis of the intellectual life. Thus in the vertebrates the primary sensory and motor neurones in number and importance are outstripped by the association neurones.

As thus sketched the development of the adjuster or central nervous element of the neuromuscular mechanism takes place in the region between the receptors and the effectors and in time after these two sets of organs have appeared. Its primary function is undoubtedly transmission involving the principle of the common path; secondarily it comes to be a repository of the effects of nervous stimulation whereby its principal function as a modifier of impulses is made possible.


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HE more usual concept of the formation of species is by slow variations so well known as the Darwinian theory, which though attacked from every point, still is and must always in the main be accepted, for without question it gives the fundamental principles of evolution as had never been done before. Yet the boundless amount of research along these lines during the last half century has developed strong new sidelights which illuminate, and in some cases compel a slightly different view of, some of the suggestions of the master, Darwin.

During the period of forty years that I have been experimenting with plant life both in bleak New England and in sunny California, extensively operating on much more than four thousand five hundred distinct species of plants, including all known economic and ornamental plant forms which are grown in the open air in temperate and semi-tropic climates, as well as many of those commonly grown in greenhouses and numerous absolutely new ones not before domesticated and on a scale never before attempted by any individual or body of individuals, numerous general principles have pressed themselves forward for discussion and observation. Only one of these can be discussed at this time, and this briefly, more as a text for further observations and experiments than as anything like a full view of this highly interesting mode of species formation.

In the first place, let me say that our so-called species are only tentative bundles of plants, no two individuals of which are exactly alike, but nearly all of which quite closely resemble each other in general outside appearances and in hereditary tendencies. Yet no one can tell just what the result will be when combinations of these inherent tendencies are crossed or subjected to any other disturbing factor or factors. Like the chemist who has new elements to work with, we may predict with some degree of accuracy what the general results will be, but any definite knowledge of the results of these combinations is far more difficult, even impossible, as the life forces of plants and animals act in infinitely more new directions than can any ordinary number of combinations of chemicals.

Only a few years ago, it was generally supposed that by crossing two somewhat different species or varieties a mongrel might be produced which might, or more likely might not, surpass its parents.


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Rh The fact that crossing was only the first step and that selection from the numerous variations secured in the second and a few succeeding generations was the real work of new plant creation had never been appreciated; and to-day its significance is not fully understood either by breeders or even by many scientific investigators along these very lines. Old tailings are constantly being worked over at great expense of time and with small profit, while the mother lode is repudiated and neglected.

Plant breeding to be successful must be conducted like architecture. Definite plans must be carefully laid for the proposed creation; suitable materials selected with judgment, and these must be securely placed in their proper order and position. No occupation requires more accuracy, foresight and skill than does scientific plant or animal breeding.

As before noted, the first generation after a cross has been made is usually a more or less complete blend of all the characteristics of both parents; not only the visible characters, but an infinite number of invisible ones are inherent and will shape the future character and destiny of the descendants, often producing otherwise unaccountable so-called mutations, saltations or sports, the selection and perpetuation of which give to new plant creations their unique forms and often priceless values, like the Burbank potato produced thirty-six years ago and which is now grown on this western coast almost to the exclusion of all others (fourteen millions of bushels per annum, besides the vast amount grown in the-eastern United States and other countries), or the Bartlett pear, Baldwin apple and navel oranges, all of which are variations selected by some keen observer. Millions of others are forever buried in oblivion for the lack of such an observer.

But in this paper I wish to call attention to a not unusual result of crossing quite distinct wild species which deserves the most careful analysis, as it seems to promise a new text for scientific investigation, especially on biometric lines. The subject was most forcibly brought to my attention twenty years ago by the singular behavior of the second-generation seedlings of raspberry-blackberry hybrids. By crossing the Siberian raspberry (Rubus cratægifolius) with our native trailing blackberry (Rubus vitifolius), a thoroughly fixed new species was summarily produced. The seedlings of this composite Rubus (named Primus), though a most perfect blend of both parents but resembling neither, never reverted either way; all the seedlings coming much more exactly like the new type than do the seedlings of any ordinary wild rubus. Many thousand plants have been raised generation after generation, all repeating themselves after the new and unique type. No botanist on earth could do otherwise than classify it if found wild as a valid new species, which it truly is, though so summarily produced by crossing.


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266 Since the Primus species was originated, numerous similar cases have attracted attention, such as my now popular Phenomenal produced by crossing the Cuthbert raspberry with our native Pacific coast blackberry, and the Logan berry, both of which, though a complete blend of two such distinct species, yet reproduce from seed as truly as any wild rubus species.

I have had also growing on my grounds for some fifteen years or more hybrids of Rubus idæus and Rubus villosus, both red and yellow varieties. All are exactly intermediate between these two very widely different species, yet both always come true intermediates from seed, generation after generation, never reverting either way.

By crossing the great African "stubble berry" (Solanum guinense) with our Pacific coast "rabbit weed" (Solanum villosum) an absolutely new species has also been produced, the fruit of which resembles in almost every particular the common blueberry (Vaccinium Pennsylvanicum), and while the fruit of neither parent species is edible, the fruit of the newly created one is most delicious and most abundantly produced, and the seedlings, generation after generation though produced by the million, still, all come as true to the new type as do either parent species to their normal type.

Still another example of this mode may be found in my experiments with opuntias. By crossing O. tuna with O. vulgaris, thousands of seedlings have been produced, all of which, in the first, second and third generations, though a well-balanced blend of the two natural species, still come as true to the newly created species as do either parent species to their own natural types.

Not only does this new mode hold true under cultivation but species are also summarily produced in a wild state by natural crossing.

The western blackcap (Rubus occidentalis) and the eastern red raspberry (Rubus strigosus) when growing contiguous, as they very commonly do in Central British America, often cross, forming an intermediate new species which sometimes sorely crowds both of the parent species, and when brought under cultivation still firmly maintains its intermediate characters, no matter how often reproduced from seed. And still further, our common "tarweed" (Madia elegans) with its beautiful large blossoms often crosses with M. saliva with its insignificant pale yellow flowers, producing a complete intermediate. I have not yet determined whether the intermediate will reproduce true from seed, but confidently expect it to do so. Similar results among wild evergreens and deciduous trees and shrubs and herbaceous plants have been frequently and forcefully brought to my attention, leaving little doubt in my own mind that the evolution of species is by more modes than some are inclined to admit.


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Rh

HE Académie Française is primarily a literary organization, and its special work is the preparation of a dictionary. But even in this enterprise it is desirable, as M. Masson points out in the document of which I propose to translate a part, to have expert assistance at hand in the matter of the meaning and use of scientific terms. It is probably for this reason that Henri Poincaré, already a member of thirty-five academies, was this year called to membership in the most celebrated of all academies. The great mathematician entered the august body with a eulogy of his predecessor, the poet Sully-Prudhomme—which task was not as strange to him as might seem at first glance, since Sully-Prudhomme was educated for a scientist and all of his work shows a scientific turn—and was received, with the customary biographical welcome, by the historian Frédéric Masson. A study by a layman and for the ears of laymen, M. Masson's address is a thoroughly popular effort; but it has a great deal that is pleasing, and not a little that is suggestive. I quote, with considerable abbreviation, from the part which deals most directly with the new academician's life and work.

You were born, a little more than half a century ago, in that dear and glorious Lorraine which has furnished this body so many men remarkable in lines of activity so diverse; so soon after we have been cruelly touched by the death of Theuriet, of Gebhart and of Cardinal Mathieu, you appear, attesting, by the exercise of a totally different genius, the inexhaustible fecundity of your native province. You come of an old race long established at Neufchâteau, and located at Nancy for a century. Of your name—Pontcaré (square bridge), rather than Poincaré (square point), for, as you have said, one might conceive a square bridge, but scarcely a square point—there have been magistrates, savants, lawyers, soldiers like the Commandant Poincaré, your great-uncle, whose tenderness for his wife and whose sad adventures M. Chuquet has narrated—like that other Poincaré, also an officer, who died for the republic in the year IX., whose son the first consul himself recommended to the ministry of war for a place in their offices, since, a corporal in the Seventh Hussars, "he had lost a leg and a thigh in one of the last battles which adorned the last campaign on the Rhine."


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268 Your grandfather was a pharmacist; it was at Nancy, in his house, opposite the ducal palace, that you came into the world; and this house, solid, massive and without ornament, is entered through an almost monumental portal whose worm-eaten posts support a broken pediment bearing the semblance of a boiling pot. Some found a bit of symbolism: the portal is poetry; the house is prose; it gives an impression of bourgeois simplicity and of settled living which is by no means trivial. Your father, a physician, was a conscientious student, a distinguished practitioner; and the faculty of Nancy, where he took his course, considered him a master of whom they were justly proud, at the same time that the working population saluted in him their benefactor. He was one of those men who, having been led by a noble curiosity into the most emotional and uncertain of professions, practise it with admirable disinterestedness and hold themselves amply repaid if they are so fortunate as to save a human life now and then. For the honor of the nation, there are many of the sort in France; but few have been able, like Dr. Poincaré, to discharge the duties of so absorbing a profession, to work in the laboratory, to teach assiduously, and at the same time to travel extensively over Europe.

Your mother was one of those alert, active women, always in motion and always busy, whose spirit of order, organization and command rules a household. She also was a native of Lorraine, of an old local family, home-loving, attached and riveted to the soil; the boys, no matter how brilliantly they had begun life, were never easy till they had returned to the home-nest to live, hunting on their estates or supervising their cultivation; two of your great-uncles joined to their rural tastes an inclination for geometry. Your mother wasted no time on such matters, finding enough to busy her in those occupations which are duties, and which, cheerfully accepted as such, become pleasures. Ah! what admirable sources of vital energy are these Frenchwomen, honest and shrewd, economical and judicious, sovereign in their own domain and disdainful of the other conquests, constantly busy at reforming the national virtue and transmitting intelligent patriotism to their children!. . . In your home you found an uncle recently graduated from the École Polytechnique. What a prestige surrounds these young men who, by a mental effort which is sometimes excessive, succeed in obtaining the first places in their generation, and to how many mistaken choices of vocation does their example lead! But with you, sir, the vocation had nothing to do with example; you were predestined to mathematics! This aptitude, in your paternal and maternal family, is transmitted in collateral lines like the throne in the House of Osman, and yourself twice heir of avuncular gifts, I am told that you have selected one of your own nephews for the precious succession.

You did not wait long to reveal your vocation, and you are justly


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Rh cited as the most precocious of infant prodigies. You were nine months old when you first saw the sky at night. You saw a star come out. You obstinately pointed out the shining spot to your mother, who was also your nurse. You discovered a second, with the same astonishment. You greeted the third, the fourth, with the same cry of joy and the same enthusiasm; it was necessary to put you to bed, you were so excited by your new occupation of star-finding. That evening brought your first contact with infinity and your first lesson in astronomy; you were the youngest professor known.

I have been told that you were a delicate, alert, charming child, spoiled and adored by your parents; a terrible illness suffered at the age of five years, as a result of which it was feared that you would never be able to speak again, left you more delicate, timid and somewhat awkward, so that you were afraid of the noisy games of the boys and preferred the society of your little sister. I do not imagine that violent sports ever tempted you, or that you ever became skilful in them. Nevertheless, you learned to hunt very large game. As soon as you learned to read, your curiosity was excited by those books of popular science which have replaced fairy stories in realistic schemes of education. You found extreme pleasure in them, and you experienced a grandiose horror in witnessing cosmic upheavals and battling with antediluvian animals. It was formerly the fashion to run after Prince Charming and awaken Sleeping Beauties. Now the child is no longer expected to make the acquaintance of those trivial personages; he must content himself with those whose skeletons have been discovered. Let me ask you: Between creatures which have really lived and of which we know nothing and never shall know anything, except that they lived, and beings which have lived only in the dreams of humanity, but which in the course of the ages have gratified us with so much beauty, grace and poetry, which are the more real, which bring more of light, of consolation, of joy? But you were not made to sit in the arm-chair of Charles Perrault.

It was in your father's house that you received from a retired teacher, a friend of your family, your first notions of things; he did not require written exercises from you; he conversed with you, talking of everything at haphazard; this encyclopedic instruction was so appropriate to your nature that when you entered the collège you at once took the first place; but this sort of work would be injurious to children of different endowment. Your memory was and still is more auditory than visual. Pronounced words engrave themselves on it. When you come back from a journey, no matter how long, you can recite the names of all the stations you have passed, if you heard them cried before your car. More than this—a character presents itself to your mind like a sound. In the evening, you can recite the numbers of


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270 all the coaches you have met in the course of the day, but you hear them, you do not see the figures. This is one of the most remarkable peculiarities of your brain, and I venture to note it because I have the unanimous testimony for it of those who know you most intimately. At the lycée of Nancy, you were superior to your comrades in every branch, and you seemed so well endowed for literary studies that one of your teachers, who is one of our best historians, would have been glad to attract you to our specialityspecialty [sic]; but when, in the fourth grade, you opened a text on geometry, the work was done. Your astonished teacher rushed to your mother and said to her: "Madam, your son will be a mathematician." And she was not particularly frightened. Mathematics, as soon as you made her acquaintance, seized you and held you. She is a tenacious mistress, with this peculiarity, that she fires all her lovers with the same impulse: the mathematician is a peripatetic. Pedestrian exercise seems necessary to him in order to stimulate thought, and, as he walks, certain mechanical gestures with which he occupies his fingers seem the indispensable auxiliaries of an intellectual labor that leaves him indifferent to the exterior world and even unconscious of it. One day, when promenading, you suddenly discovered that you were carrying in your hand a wicker cage. You were prodigiously surprised. When, where, how had your hand plucked this cage, which was new and fortunately empty? You had no idea, and retracing your steps, you walked until you found on the sidewalk the stock of a basket-maker whom you had innocently despoiled. Such phenomena are very common with you; they will become, if they are not already so, as celebrated as those attributed to Lagrange, to Kant, to Ampere. You might be in worse company.

You were, nevertheless, at times, a child who liked pleasure and games, but you invented your own amusements. You played at railroad or diligence with a map or a guide in reach, and thus you learned geography. You put history into dramas and comedies; at sixteen years you had written a five-act tragedy in verse, and you would not have been a son of Lorraine if the heroine had not been Joan of Arc. Even charades had a charm for you. Are they not problems? The war interrupted these games. You were sixteen years old; your age and your health prevented your mingling with the combatants, but you tried to make yourself useful; every day you accompanied your father to the hospital and served as his secretary; you were so eager to learn the news that, in order to read them in the only papers that were accessible to you, you learned German. The war must have matured you; it certainly left its trace upon you; but it did not change your life. To the men of the generation preceding yours, it brought a definite conversion with introspection. You have read Sully-Prudhomme's verses entitled "Repentance." In them he confesses the error into which the generosity of his heart had drawn him and in which the


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Rh discourses of the rhetoricians had maintained him; in order to carry out designs which were unworthy and shameful, these gentry resort to sonorous words to lull a careless people to security; and when the nation awakens and finds herself rolling into the abyss, she cries out treason but is unable to distinguish the traitors. So Sully-Prudhomme had detested war and shown himself rather disdainful of soldiers. Then he learned from his own experience that any one who chooses can not be a soldier, that it is one thing to deliver philosophic harangues and another to submit one's physical and moral being to monotonous regulations and entire self-effacement; he learned—and the lesson cost him dear—that in order to possess the right to think, one must have conquered first the right to live; that it is folly which would be ridiculous if it did not bring such despair to profess humanitarianism when all of Europe is under arms; and that, however inelegant the solution may appear, there is but one, if a people intends to maintain its nationality, guard its independence, continue its race, possess its territory, speak its language—and the solution is to be strong enough to defend them.

You lived your life, sir, under the yoke of the victorious enemy. It was in a city occupied by the Germans that you resumed and continued your studies. You were thoroughly successful in them; but the joy was doubled for you by the fact that your public success coincided with the evacuation of Nancy. As our dear late colleague Émile Gebhart has told us, it was in a hall filled with the joy of deliverance that you received your last scholastic honors. You held the first rank, a native of the city and ten times a prize-winner. You carried off the prize in mathematics from all your rivals, from Paris and the departments; it depended on you alone to enter the School of Forestry second on the list of appointees; this would have been another glory for Nancy, but you refused to go further with the school than to leave your visiting-card; you were distrustful of the fallacious dryads who delight in troubling the absent-minded.

The next year you presented yourself as a candidate at the École Polytechnique and at the same time at the École Normale; for the latter you stood number five, for the former number one. Which of the two great schools would you choose? That which decided your choice, more even than the familiar memories, than the temptation of the uniform and the glory of the sergeant-major's chevrons, was it not, tell us, the groaning of the mutilated fatherland? But you never reached the point of entering upon a military career. Your scientific bent showed itself so brilliantly at the school that there was no question of another sort of glory; your residence there is a matter of piously transmitted tradition. It is related that you attended your classes, at least in mathematics, without taking a note, without reading or even collecting the mimeographed sheets which reproduce the professor's lecture. Your method