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A Critique of the Theory of Evolution/III

CHAPTER III

THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM

 * 1. The Cellular Basis of Organic Evolution and Heredity
 * 2. The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes
 * 3. The Four Great Linkage Groups of Drosophila Ampelophila
 * a. Group I.
 * b. Group II.
 * c. Group III.
 * d. Group IV.
 * 4. Localization of Factors in the Chromosomes
 * a. The Evidence from Sex Linked Inheritance
 * b. The Evidence from Interference
 * c. The Evidence from Non-Disjunction
 * 5. How Many Genetic Factors are there in the Germ-plasm of a Single Individual?
 * 6. Conclusions

The discovery that Mendel made with edible peas concerning heredity has been found to apply everywhere throughout the plant and animal kingdoms—to flowering plants, to insects, snails, crustacea, fishes, amphibians, birds, and mammals (including man).

There must be something that these widely separated groups of plants and animals have in common—some simple mechanism perhaps—to give such definite and orderly series of results. There is, in fact, a mechanism, possessed alike by animals and plants, that fulfills every requirement of Mendel's principles.

In order to appreciate the full force of the evidence, let me first pass rapidly in review a few familiar, historical facts, that preceded the discovery of the mechanism in question.

Throughout the greater part of the last century, while students of evolution and of heredity were engaged in what I may call the more general, or, shall I say, the grosser aspects of the subject, there existed another group of students who were engaged in working out the minute structure of the material basis of the living organism. They found that organs such as the brain, the heart, the liver, the lungs, the kidneys, etc., are not themselves the units of structure, but that all these organs can be reduced to a simpler unit that repeats itself a thousand-fold in every organ. We call this unit a cell (fig. 45).

The egg is a cell, and the spermatozoon is a cell. The act of fertilization is the union of two cells (fig. 47, upper figure). Simple as the process of fertilization appears to us today, its discovery swept aside a vast amount of mystical speculation concerning the rôle of the male and of the female in the act of procreation.

Within the cell a new microcosm was revealed. Every cell was found to contain a spherical body called the nucleus (fig. 46a). Within the nucleus is a network of fibres, a sap fills the interstices of the network. The network resolves itself into a definite number of threads at each division of the cell (fig. 46 b-e). These threads we call chromosomes. Each species of animals and plants possesses a characteristic number of these threads which have a definite size and sometimes a specific shape and even characteristic granules at different levels. Beyond this point our strongest microscopes fail to penetrate. Observation has reached, for the time being, its limit.

The story is taken up at this point by a new set of students who have worked in an entirely different field. Certain observations and experiments that we have not time to consider now, led a number of biologists to conclude that the chromosomes are the bearers of the hereditary units. If so, there should be many such units carried by each chromosome, for the number of chromosomes is limited while the number of independently inherited characters is large. In Drosophila it has been demonstrated not only that there are exactly as many groups of characters that are inherited together as there are pairs of chromosomes, but even that it is possible to locate one of these groups in a particular chromosome and to state the relative position there of the factors for the characters. If the validity of this evidence is accepted, the study of the cell leads us finally in a mechanical, but not in a chemical sense, to the ultimate units about which the whole process of the transmission of the hereditary factors centers.

But before plunging into this somewhat technical matter (that is difficult only because it is unfamiliar), certain facts which are familiar for the most part should be recalled, because on these turns the whole of the subsequent story.

The thousands of cells that make up the cell-state that we call an animal or plant come from the fertilized egg. An hour or two after fertilization the egg divides into two cells (fig. 47). Then each half divides again. Each quarter next divides. The process continues until a large number of cells is formed and out of these organs mould themselves.

At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance.

At an early stage in the development of the animal certain cells are set apart to form the organs of reproduction. In some animals these cells can be identified early in the cleavage (fig. 48).

The reproductive cells are at first like all the other cells in the body in that they contain a full complement of chromosomes, half paternal and half maternal in origin (fig. 49). They divide as do the other cells of the body for a long time (fig. 49, upper row). At each division each chromosome splits lengthwise and its halves migrate to opposite poles of the spindle (fig. 49 c).

But there comes a time when a new process appears in the germ cells (fig 49 e-h). It is essentially the same in the egg and in the sperm cells. The discovery of this process we owe to the laborious researches of many workers in many countries. The list of their names is long, and I shall not even attempt to repeat it. The chromosomes come together in pairs (fig. 49 a). Each maternal chromosome mates with a paternal chromosome of the same kind.

Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the divisions the double chromosomes separate so that each resulting cell comes to contain some maternal and some paternal chromosomes, i.e. one or the other member of each pair. At the other division each chromosome simply splits as in ordinary cell division.

The upshot of the process is that the ripe eggs (fig. 51) and the ripe spermatozoa (fig. 50) come to contain only half the total number of chromosomes.

When the eggs are fertilized the whole number of chromosomes is restored again.

If the factors in heredity are carried in the chromosomes and if the chromosomes are definite structures, we should anticipate that there should be as many groups of characters as there are kinds of chromosomes. In only one case has a sufficient number of characters been studied to show whether there is any correspondence between the number of hereditary groups of characters and the number of chromosomes. In the fruit fly, Drosophila ampelophila, we have found about 125 characters that are inherited in a perfectly definite way. On the opposite page is a list of some of them.

It will be observed in this list that the characters are arranged in four groups, Groups I, II, III and IV. Three of these groups are equally large or nearly so; Group IV contains only two characters. The characters are put into these groups because in heredity the members of each group tend to be inherited together, i.e., if two or more enter the cross together they tend to remain together through subsequent generations. On the other hand, any member of one group is inherited entirely independently of any member of the other groups; in the same way as Mendel's yellow-green pair of characters is inherited independently of the round-wrinkled pair.

If the factors for these characters are carried by the chromosomes, then we should expect that those factors that are carried by the same chromosome would be inherited together, provided the chromosomes are definite structures in the cell.

In the chromosome group of Drosophila, (fig. 52) there are four pairs of chromosomes, three of nearly the same size and one much smaller. Not only is there agreement between the number of hereditary groups and the number of the chromosomes, but even the size relations are the same, for there are three great groups of characters and three pairs of large chromosomes, and one small group of characters and one pair of small chromosomes.

The following description of the characters of the wild fly may be useful in connection with the account of the modifications of these characters that appear in the mutants.

The head and thorax of the wild fly are grayish-yellow, the abdomen is banded with alternate stripes of yellow and black. In the male, (fig. 4 to right), there are three narrow bands and a black tip. In the female there are five black bands (fig. 4 to left). The wings are gray with a surface texture of such a kind that at certain angles they are iridescent. The eyes are a deep, solid, brick-red. The minute hairs that cover the body have a very definite arrangement that is most obvious on the head and thorax. There is a definite number of larger hairs called bristles or chaetae which have a characteristic position and are used for diagnostic purposes in classifying the species. On the foreleg of the male there is a comb-like organ formed by a row of bristles; it is absent in the female. The comb is a secondary sexual character, and it is, so far as known, functionless.

Some of the characters of the mutant types are shown in figures 53, 54, 55, 56. The drawing of a single fly is often used here to illustrate more than one character. This is done to economize space, but of course there would be no difficulty in actually bringing together in the same individual any two or more characters belonging to the same group (or to different groups). Without colored figures it is not possible to show many of the most striking differences of these mutant races; at most dark and light coloring can be indicated by the shading of the body, wings, or eyes.

Group I

In the six flies drawn in figure 53 there are shown five different wing characters. The first of these types (a) is called cut, because the ends of the wings look as though they had been cut to a point. The antennae are displaced downward and appressed and their bristle-like aristae are crumpled.

The second figure (b) represents a fly with a notch in the ends of the wings. This character is dominant, but the same factor that produces the notch in the wings is also a recessive lethal factor; because of this latter effect of the character no males of this race exist, and the females of the race are never pure but hybrid. Every female with notch wings bred to a wild male, will produce in equal numbers notch winged daughters and daughters with normal wings. There will be half as many sons as daughters. The explanation of this peculiar result is quite simple. Every notch winged female has one X chromosome that carries the factor for notch and one X chromosome that is "normal". Daughters receiving the former chromosomes are notched because the factor for notch is dominant, but they are not killed since the lethal effect of the notch factor is recessive to the normal allelomorph carried by the other chromosome that the daughters get from their father. This normal factor is recessive for notch but dominant for life. This same figure (b) is used here to show three other sex linked characters. The spines on the thorax are twisted or kinky, which is due to a factor called "forked". The effect is best seen on the thorax, but all spines on the body are similarly modified; even the minute hairs are also affected. Ruby eye color might be here represented—if the eyes in the figure were colored. The lighter color of the body and antennae is intended to indicate that the character tan is also present. The light color of the antennae is the most certain way of identifying tan. The tan flies are interesting because they have lost the positive heliotropism that is so marked a feature in the behavior of D. ampelophila. As this peculiarity of the tan flies is inherited like all the other sex linked characters, it follows that when a tan female is bred to a wild male all the sons inherit the recessive tan color and indifference to light, while the daughters show the dominant sex linked character of their father, i.e., they are "gray", and go to the light. Hence when such a brood is disturbed the females fly to the light, but the males remain behind.

One of the first mutants that appeared in D. ampelophila was called rudimentary on account of the condition of the wings (c). The same mutation has appeared independently several times. In the drawing (c) the dark body color is intended to indicate "sable" and the lighter color of the eyes is intended to indicate eosin. This eye color, which is an allelomorph of white, is also interesting because in the female the color is deeper than in the male. In other cases of sex linked factors the character is the same in the two sexes.

In the fourth figure (d) the third and fourth longitudinal veins of the wing are fused into one vein from the base of the wing to the level of the first cross-vein and in addition converge and meet near their outer ends. The shape of the eye is represented in the figure as different from the normal, due to another factor called "bar". This is a dominant character, the hybrid condition being also narrow, but not so narrow as the pure type. Vermilion eye color might also be here represented—due to a factor that has appeared independently on several occasions.

In the fifth figure (e) the wings are shorter and more pointed than in the wild fly. This character is called miniature. The light color of the drawing may be taken to represent yellow body color, and the light color of the eye white eye color.

In the last figure (f) the wings are represented as pads, essentially in the same condition that they are in when the fly emerges from the pupa case. Not all the flies of this stock have the wings in this condition; some have fully expanded wings that appear normal in all respects. Nevertheless, about the same percentage of offspring show the pads irrespective of whether the parents had pads or expanded wings.

The flies of this stock show, however, another character, which is a product of the same factor, and which is constant, i.e., repeated in all individuals. The two bristles on the sides of the thorax are constantly absent in this race. The lighter color of the eye in the figure may be taken to indicate buff—a faint yellowish color. The factor for this eye color is another allelomorph of white.

There are many other interesting characters that belong to the first group, such as abnormal abdomen, short legs, duplication of the legs, etc. In fact, any part of the body may be affected by a sex-linked factor.

Group II

In the first figure (a) of figure 54 that contains members of Group II the wings are almost entirely absent or "vestigial". This condition arose at a single step and breeds true, although it appears to be influenced to some extent by temperature, also by modifiers that sometimes appear in the stock. Purple eye color belongs in Group II; it resembles the color of the eye of the wild fly but is darker and more translucent.

In the second figure (b) the wing is again long and narrow and sometimes bent back on itself, as shown here. In several respects the wing resembles strap (d) but seems to be due to another factor, called antler, insufficiently studied as yet.

In the third figure (c) the wings turn up at the end. This is brought about by the presence of the factor called jaunty.

In the fourth figure the wings are long and narrow and several of the veins are unrepresented. This character, "strap", is very variable and has not yet been thoroughly studied. On the thorax there is a deep black mark called trefoil. Even in the wild fly there is a three pronged mark on the thorax present in many individuals. Trefoil is a further development and modification of this mark and is due to a special factor.

In the fifth figure (e) the wings are arched. The factor is called arc. The dark color of the body, and especially of the wings, indicates the factor for black.

The sixth figure (f) shows the wings "curved" downwards. In addition there is present a minute black speck at the base of each wing, due to another factor called speck.

In the seventh figure (g) the wing is truncate. Its end is obliquely squared instead of rounded; it may be longer than the body, or shorter when other modifying factors are present. The mutation that produces this type of wing is of not infrequent occurrence. It has been shown by Muller and Altenburg that there are at least two factors that modify this character—the chief factor is present in the second chromosome; alone it produces the truncate wing in only a certain percentage of cases, but when the modifiers are also present about ninety percent of the individuals may show the truncate condition of the wing. But the presence of these factors makes the stock very infertile, so that it is difficult to maintain.

In the eighth figure (h) the legs are shortened owing to the absence of a segment of the tarsus. The stock is called dachs—a nickname given to it because the short legs suggested the dachshund.

Group III

In figure 55, (a), a mutant type called bithorax is shown. The old metathorax is replaced by another mesothorax thrust in between the normal mesothorax and the abdomen. It carries a pair of wings that do not completely unfold. On this new mesothorax the characteristic arrangement of the bristles is shown. Thus at a single step a typical region of the body has doubled. The character is recessive.

The size of the adult fly of D. ampelophila varies greatly according to the amount of nourishment obtained by the larva. After the fly emerges its size remains nearly constant, as in many insects. Two races have, however, been separated by Bridges that are different in size as a result of a genetic factor. The first of these, called dwarf, is represented by figure 55, (b).

The race is minute, although of course its size is variable, depending on food and other conditions. The same figure shows the presence of another factor, "sooty", that makes the fly very dark. Maroon eye color might be here represented, due to still another factor.

In the third figure (c) the other mutation in size is shown. It is called "giant". The flies are twice the size of wild flies. An eye color, called peach, might here be represented. It is an allelomorph of pink.

In the fourth figure (d) the mutant called dichaete is shown. It is characterized by the absence of two of the bristles on the thorax. Other bristles may also be absent, but not so constantly as the two just mentioned. Another effect of the same factor is the spread-out condition of the wings. The very dark eye color in this figure may be taken to indicate the presence of another factor, "sepia", which causes the eyes to assume a brown color that becomes black with age. Most of the other mutations in eye color that have occurred tend to give a lighter color: this one, which is also recessive, makes the eye darker.

In the fifth figure (e) the color of the darkest fly is due to a factor called ebony, which is an allelomorph of sooty.

In the sixth figure (f) the wings are beaded, i.e., the margin is defective at intervals, giving a beaded-like outline to the wings. This condition is very variable and much affected by other factors that influence the shape of the wings. The lighter eye color of the drawing may be taken to represent pink.

In the seventh figure (g) the wings are curled up over the back. This is a recessive character.

Group IV

Only two mutants have been obtained that do not belong to any of the preceding groups; these are put together in Group IV. It has been shown that they are linked to each other and the linkage is so close that it has thus far been impossible to obtain the dominant recessive. One of these mutants, called "eyeless" (fig. 56, a, a$1$), is variable—the eyes are often entirely absent or represented by one or more groups of ommatidia. The outline of the original eye, so to speak, is strongly marked out and its area might be called a rudimentary organ, if such a statement has any meaning here.

The other figure (b) represents "bent", so called from the shape of the wings. This mutant is likewise very variable, often indistinguishable from the wild type, yet when well developed strikingly different from any other mutant.

This brief account of a few of the mutant races that can be most easily represented by uncolored figures will serve to show how all parts of the body may change, some of the changes being so slight that they would be overlooked except by an expert, others so great that in the character affected the flies depart far from the original species.

''It is important to note that mutations in the first chromosome are not limited to any part of the body nor do they affect more frequently a particular part. The same statement holds equally for all of the other chromosomes. In fact, since each factor may affect visibly several parts of the body at the same time there are no grounds for expecting any special relation between a given chromosome and special regions of the body. It can not too insistently be urged that when we say a character is the product of a particular factor we mean no more than that it is the most conspicuous effect of the factor.''

If, then, as these and other results to be described point to the chromosomes as the bearers of the Mendelian factors, and if, as will be shown presently, these factors have a definite location in the chromosomes it is clear that the location of the factors in the chromosomes bears no spatial relation to the location of the parts of the body to each other.

The Evidence from Sex Linked Inheritance

When we follow the history of pairs of chromosomes we find that their distribution in successive generations is paralleled by the inheritance of Mendelian characters. This is best shown in the sex chromosomes (fig. 57). In the female there are two of these chromosomes that we call the X chromosomes; in the male there are also two but one differs from those of the female in its shape, and in the fact that it carries none of the normal allelomorphs of the mutant factors. It is called the Y chromosome.

The course followed by the sex chromosomes and that by the characters in the case of sex linked inheritance are shown in the next diagram of Drosophila illustrating a cross between a white eyed male and a red eyed female.

The first of these represents a cross between a white eyed male and a red eyed female (fig. 58, top row). The X chromosome in the male is represented by an open bar, the Y chromosome is bent. In the female the two X chromosomes are black. Each egg of such a female will contain one "black" X after the polar bodies have been thrown off. In the male there will be two classes of sperm—the female-producing, carrying the (open) X, and the male-producing, carrying the Y chromosome. Any egg fertilized by an X bearing sperm will produce a female that will have red eyes because the X (black) chromosome it gets from the mother carries the dominant factor for red. Any egg fertilized by a Y-bearing sperm will produce a male that will also have red eyes because he gets his (black) X chromosome from his mother.

When, then, these two F$1$ flies (second row) are inbred the following combinations are expected. Each egg will contain a black X (red eye producing) or a white X (white eye producing) after the polar bodies have been extruded. The male will produce two kinds of sperms, of which the female producing will contain a black X (red eye producing). Since any egg may by chance be fertilized by any sperm there will result the four classes of individuals shown on the bottom row of the diagram. All the females will have red eyes, because irrespective of the two kinds of eggs involved all the female-producing sperm carry a black X. Half of the males have red eyes because half of the eggs have had each a red-producing X chromosome. The other half of the males have white eyes, because the other half of the eggs had each a white-producing X chromosome. Other evidence has shown that the Y chromosome of the male is indifferent, so far as these Mendelian factors are concerned.

The reciprocal experiment is illustrated in figure 59. A white eyed female is mated to a red eyed male (top row). All the mature eggs of such a female contain one white-producing X chromosome represented by the open bar in the diagram. The red eyed male contains female-producing X-bearing sperm that carry the factor for red eye color, and male-producing Y chromosomes. Any egg fertilized by an X-bearing sperm will become a red eyed female because the X chromosome that comes from the father carries the dominant factor for red eye color. Any egg fertilized by a Y-bearing sperm will become a male with white eyes because the only X chromosome that the male contains comes from his mother and is white producing.

When these two F$1$ flies are inbred (middle row) the following combinations are expected. Half the eggs will contain each a white producing X chromosome and half red producing. The female-producing sperms will each contain a white X and the male-producing sperms will each contain an indifferent Y chromosome. Chance meetings of egg and sperm will give the four F$2$ classes (bottom row). These consist of white eyed and red eyed females and white eyed and red eyed males. The ratio here is 1:1 and not three to one (3:1) as in other Mendelian cases. But Mendel's law of segregation is not transgressed, as the preceding analysis has shown; for, the chromosomes have followed strictly the course laid down on Mendel's principle for the distribution of factors. The peculiar result in this case is due to the fact that the F$1$ male gets his single factor for eye color from his mother only and it is linked to or contained in a body (the X chromosome) that is involved in producing the females, while the mate of this body—the Y chromosome—is indifferent with regard to these factors, yet active as a mate to X in synapsis.

In man there are several characters that show exactly this same kind of inheritance. Color blindness, or at least certain kinds of color blindness, appear to follow the same scheme. A color blind father transmits through his daughters his peculiarity to half of his grandsons, but to none of his grand-daughters (fig. 38A). The result is the same as in the case of the white eyed male of Drosophila. Color blind women are rather unusual, which is expected from the method of inheritance of this character, but in the few known cases where such color blind women have married normal husbands the sons have inherited the peculiarity from the mother (fig. 38B). Here again the result is the same as for the similar combination in Drosophila.

In man the sex formula appears to be XX for the female and XO for the male (fig. 60), and since the relation is essentially the same as that in Drosophila the chromosome explanation is the same. According to von Winiwarter there are 48 chromosomes in the female and 47 in the male (fig. 61). After the extrusion of the polar bodies there are 24 chromosomes in the egg. In the male at one of the two maturation divisions the X chromosome passes to one pole undivided (fig. 61, C). In consequence there are two classes of sperms in man; female producing containing 24 chromosomes, and male producing containing 23 chromosomes. If the factor for color blindness is carried by the X chromosome its inheritance in man works out on the same chromosome scheme and in the same way as does white eye color (or any other sex linked character) in the fly, for the O sperm in man is equivalent to the Y sperm in the fly.

In these cases we have been dealing with a single pair of characters. Let us now take a case where two pairs of sex linked characters enter the cross at the same time, and preferably a case where the two recessives enter the cross from the same parent.

If a female with white eyes and yellow wings is crossed to a wild male with red eyes and gray wings (fig. 62), the sons are yellow and have white eyes and the daughters are gray and have red eyes. If two F$1$ flies are mated they will produce the following classes.

Not only have the two grandparental combinations reappeared, but in addition two new combinations, viz., grey white and yellow red. The two original combinations far exceed in numbers the new or exchange combinations. If we follow the history of the X chromosomes we discover that the larger classes of grandchildren appear in accord with the way in which the X chromosomes are transmitted from one generation to the next.

The smaller classes of grandchildren, the exchange combinations or cross-overs, as we call them, can be explained by the assumption that at some stage in their history an interchange of parts has taken place between the chromosomes. This is indicated in the diagrams.

The most important fact brought out by the experiment is that the factors that went in together tend to stick together. It makes no difference in what combination the members of the two pairs of characters enter, they tend to remain in that combination.

If one admits that the sex chromosomes carry these factors for the sex-linked characters—and the evidence is certainly very strong in favor of this view—it follows necessarily from these facts that at some time in their history there has been an interchange between the two sex chromosomes in the female.

There are several stages in the conjugation of the chromosomes at which such an interchange between the members of a pair might occur. There is further a small amount of direct evidence, unfortunately very meagre at present, showing that an interchange does actually occur.

At the ripening period of the germ cell the members of each pair of chromosomes come together (fig. 49, e). In several forms they have been described as meeting at one end and then progressively coming to lie side by side as shown in fig. 63, e, f, g, h, i. At the end of the process they appear to have completely united along their length (fig. 63, j, k, l). It is always a maternal and a paternal chromosome that meet in this way and always two of the same kind. It has been observed that as the members of a pair come together they occasionally twist around each other (fig. 63, g, l, and 64, and 65). In consequence a part of one chromosome comes to be now on one side and now on the other side of its mate.

When the chromosomes separate at the next division of the germ cell the part on one side passes to one pole, the part on the other to the opposite pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this time there must result an interchange of pieces where they were crossed over each other.

Janssens has found at the time of separation evidence in favor of the view that some such interchange probably takes place.

We find this same process of interchange of characters taking place in each of the other three groups of Drosophila. An example will show this for the Group II.

If a black vestigial male is crossed to a gray long-winged female (fig. 66) the offspring are gray long. If an F$1$ female is back-crossed to a black vestigial male the following kinds of flies are produced:

The combinations that entered are more common in the F$2$ generations than the cross-over classes, showing that there is linkage of the factors that entered together.

Another curious fact is brought out if instead of back-crossing the F$1$ female we back-cross the F$1$ male to a black vestigial female. Their offspring are now of only two kinds, black vestigial and gray long. This means that in the male there is no crossing-over or interchange of pieces. This relation holds not only for the Group II but for all the other groups as well.

Why interchange takes place in the female of Drosophila and not in the male we do not know at present. We might surmise that when in the male the members of a pair come together they do not twist around each other, hence no crossing-over results.

Crossing-over took place between white and yellow only once in a hundred times. Other characters show different values, but the same value under the same conditions is obtained from the same pair of characters.

If we assume that the nearer together the factors lie in the chromosome the less likely is a twist to occur between them, and conversely the farther apart they lie the more likely is a twist to occur between them, we can understand how the linkage is different for different pairs of factors.

On this basis we have made out chromosomal maps for each chromosome (fig. 67). The diagram indicates those loci that have been most accurately placed.

The Evidence from Interference

There is a considerable body of information that we have obtained that corroborates the location of the factors in the chromosome. This evidence is too technical to take up in any detail, but there is one result that is so important that I must attempt to explain it. If, as I assume, crossing over is brought about by twisting of the chromosomes, and if owing to the material of the chromosomes there is a most frequent distance of internode, then, when crossing over between nodes takes place at same level at a-b in figure 68, the region on each side of that point, a to A and b to B, should be protected, so to speak, from further crossing over. This in fact we have found to be the case. No other explanation so far proposed will account for this extraordinary relation.

What advantage, may be asked, is there in obtaining numerical data of this kind? It is this:—whenever a new character appears we need only determine in which of the four groups it lies and its distance from two members within that group. With this information we can predict with a high degree of probability what results it will give with any other member of any group. Thus we can do on paper what would require many months of labor by making the actual experiment. In a word we can predict what will happen in a situation where prediction is impossible without this numerical information.

The Evidence from Non-Disjunction

In the course of the work on Drosophila exceptions appeared in one strain where certain individuals did not conform to the scheme of sex linked inheritance. For a moment the hypothesis seemed to fail, but a careful examination led to the suspicion that in this strain something had happened to the sex chromosomes. It was seen that if in some way the X chromosomes failed to disjoin in certain eggs, the exceptions could be explained. The analysis led to the suggestion that if the Y chromosome had got into the female line the results would be accounted for, since its presence there would be expected to cause this peculiar non-disjunction of the X chromosomes.

That this was the explanation was shown when the material was examined. The females that gave these results were found by Bridges to have two X's and a Y chromosome.

The normal chromosome group of the female is shown in figure 52 and the chromosome group of one of the exceptional females is shown in figure 69. In a female of this kind there are three sex chromosomes X X Y which are homologous in the sense that in normal individuals the two present are mates and separate at the reduction division. If in the X X Y individual X and X conjugate and separate at reduction and the unmated Y is free to move to either pole of the spindle, two kinds of mature eggs will result, viz., X and XY. If, on the other hand, X and Y conjugate and separate at reduction and the remaining X is free to go to either pole, four kinds of eggs will result—XY—X—XX—Y. As a total result four kinds of eggs are expected: viz. many XY and X eggs and a few XX and Y eggs.

These four kinds of eggs may be fertilized either by female-producing sperms or male-producing sperms, as indicated in the diagram (fig. 70).

If such an XXY female carried white bearing Xs (open X in the figures), and the male carried a red bearing X (black X in the figures) it will be seen that there should result an exceptional class of sons that are red, and an exceptional class of daughters that are white. Tests of these exceptions show that they behave subsequently in heredity as their composition requires. Other tests may also be made of the other classes of offspring. Bridges has shown that they fulfill all the requirements predicted. Thus a result that seemed in contradiction with the chromosome hypothesis has turned out to give a brilliant confirmation of that theory both genetically and cytologically.

In passing I invite your attention to a speculation based on our maps of the chromosomes—a speculation which I must insist does not pretend to be more than a guess but has at least the interest of being the first guess that we have ever been in position to make as to how many factors go towards the makeup of the germ plasm.

We have found practically no factors less than .04 of a unit apart. If our map includes the entire length of the chromosomes and if we assume factors are uniformly distributed along the chromosome at distances equal to the shortest distance yet observed, viz. .04, then we can calculate roughly how many hereditary factors there are in Drosophila. The calculation gives about 7500 factors. The reader should be cautioned against accepting the above assumptions as strictly true, for crossing-over values are known to differ according to different environmental conditions (as shown by Bridges for age), and to differ even in different parts of the chromosome as a result of the presence of specific genetic factors (as shown by Sturtevant). Since all the chromosomes except the X chromosomes are double we must double our estimate to give the total number of factors, but the half number is the number of the different kinds of factors of Drosophila.

I have passed in review a long series of researches as to the nature of the hereditary material. We have in consequence of this work arrived within sight of a result that seemed a few years ago far beyond our reach. The mechanism of heredity has, I think, been discovered—discovered not by a flash of intuition but as the result of patient and careful study of the evidence itself.

With the discovery of this mechanism I venture the opinion that the problem of heredity has been solved. We know how the factors carried by the parents are sorted out to the germ cells. The explanation does not pretend to state how factors arise or how they influence the development of the embryo. But these have never been an integral part of the doctrine of heredity. The problems which they present must be worked out in their own field. So, I repeat, the mechanism of the chromosomes offers a satisfactory solution of the traditional problem of heredity.


text: Index:Life and Works of Abraham Lincoln, v5.djvu

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 would favor the next largest sum, so that the vote would be greater, and so on down to the largest sum that is favored by a majority. If the voting began with the smallest sum, every one would be willing to pay that amount, and it might be adopted and thus cut off voting on the other propositions, whereas a majority would prefer authorizing the committee to spend a larger amount. On the other hand, suppose the committee was being authorized to sell a building for a blank amount: here it is evident that there would be more in favor of the large sum than of the small one. So to get at the wish of the assembly the voting should begin with the smallest sum proposed; all who are willing to sell for that amount, and some additional ones, will be willing to sell for the next larger sum; and so the smallest sum for which the majority is willing to sell will be gradually reached.

It is sometimes convenient to create a blank, as in the following example: A resolution is pending requesting the proper authorities to prohibit the erection of wooden buildings north of A street, and an amendment to strike out A and insert B, and an amendment of the second degree to strike out B and insert C, have been made. The debate developing the fact that several other streets have their advocates, the best course is for the chair to state that, if there is no objection, the motion would be treated as having a blank for the name of


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 the street, and that A, B, and C have been proposed for filling the blank. In this way other names could be suggested and they would be voted on successively beginning with the one that made the prohibited area the largest, and continuing down until one was reached that could get a majority in its favor. If objection is made to leaving a blank for the name, the chair may put the question without waiting for a motion, or any one may move, as an incidental motion, that a blank be created for the name of the street. This motion is undebatable, and cannot be amended, but it may be moved to fill the blank by ballot or in any other way.

The blanks in a resolution should be filled usually before voting on the resolution. But sometimes, when a large majority is opposed to the resolution, the previous question is ordered without waiting for the blanks to be filled, thus stopping debate and further amendment, and bringing the assembly at once to a vote on the resolution. Under such circumstances the resolution would usually be rejected. But should it be adopted, it would be necessary to fill the blanks in the skeleton resolution before any other than privileged business would be in order.

The method adopted in filling blanks has sometimes a great advantage over ordinary amendment. In amending, the last one proposed is the first one voted on, whereas in


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A Critique of the Theory of Evolution/IV

CHAPTER IV

SELECTION AND EVOLUTION

 * 1. The Theory of Natural Selection
 * 2. How has Selection in Domesticated Animals and Plants brought about its Results?
 * 3. Are Factors Changed through Selection?
 * 4. How does Natural Selection Influence the course of Evolution?
 * 5. Conclusions

Darwin's Theory of Natural Selection still holds today first place in every discussion of evolution, and for this very reason the theory calls for careful scrutiny; for it is not difficult to show that the expression "natural selection" is to many men a metaphor that carries many meanings, and sometimes different meanings to different men. While I heartily agree with my fellow biologists in ascribing to Darwin himself, and to his work, the first place in biological philosophy, yet recognition of this claim should not deter us from a careful analysis of the situation in the light of work that has been done since Darwin's time.

In his great book on the Origin of Species, Darwin tried to do two things: first, to show that the evidence bearing on evolution makes that explanation probable. No such great body of evidence had ever been brought together before, and it wrought, as we all know, a revolution in our modes of thinking.

Darwin also set himself the task of showing how evolution might have taken place. He pointed to the influence of the environment, to the effects of use and disuse, and to natural selection. It is to the last theory that his name is especially attached. He appealed to a fact familiar to everyone, that no two individuals are identical and that some of the differences that they show are inherited. He argued that those individuals that are best suited to their environment are the most probable ones to survive and to leave most offspring. In consequence their descendants should in time replace through competition the less well-adapted individuals of the species. This is the process Darwin called natural selection, and Spencer the survival of the fittest.

Stated in these general terms there is nothing in the theory to which anyone is likely to take exception. But let us examine the argument more critically.

If we measure, or weigh, or classify any character shown by the individuals of a population, we find differences. We recognize that some of the differences are due to the varied experiences that the individuals have encountered in the course of their lives, i.e. to their environment, but we also recognize that some of the differences may be due to individuals having different inheritances—different germ plasms. Some familiar examples will help to bring home this relation.

If the leaves of a tree are arranged according to size (fig. 71), we find a continuous series, but there are more leaves of medium size than extremes. If a lot of beans be sorted out according to their weights, and those between certain weights put into cylinders, the cylinders, when arranged according to the size of the beans, will appear as shown in figure 72. An imaginary line running over the tops of the piles will give a curve (fig. 73) that corresponds to the curve of probability (fig. 74).

If we stand men in lines according to their height (fig. 75) we get a similar arrangement.

The differences in size shown by the individual beans or by the individual men are due in part to heredity, in part to the environment in which they have developed. This is a familiar fact of almost every-day observation. It is well shown in the following example. In figure 76 the two boys and the two varieties of corn, which they are holding, differ in height. The pedigrees of the boys (fig. 77) make it probable that their height is largely inherited and the two races of corn are known to belong to a tall and a short race respectively. Here, then, the chief effect or difference is due to heredity. On the other hand, if individuals of the same race develop in a favorable environment the result is different from the development in an unfavorable environment, as shown in figure 78. Here to the right the corn is crowded and in consequence dwarfed, while to the left the same kind of corn has had more room to develop and is taller.

Darwin knew that if selection of particular kinds of individuals of a population takes place the next generation is affected. If the taller men of a community are selected the average of their offspring will be taller than the average of the former population. If selection for tallness again takes place, still taller men will on the average arise. If, amongst these, selection again makes a choice the process would, he thought, continue (fig. 79).

We now recognize that this statement contains an important truth, but we have found that it contains only a part of the truth. Any one who repeats for himself this kind of selection experiment will find that while his average class will often change in the direction of his selection, the process slows down as a rule rather suddenly (fig. 80). He finds, moreover, that the limits of variability are not necessarily transcended as the process continues even although the average may for a while be increased. More tall men may be produced by selection of this kind, but the tallest men are not necessarily any taller than the tallest in the original population.

Selection, then, has not produced anything new, but only more of certain kinds of individuals. Evolution, however, means producing more new things, not more of what already exists.

Darwin seems to have thought that the range of variation shown by the offspring of a given individual about that type of individual would be as wide as the range shown by the original population (fig. 79), but Galton's work has made it clear that this is not the case in a general or mixed population. If the offspring of individuals continued to show, as Darwin seems to have thought, as wide a range on each side of their parents' size, so to speak, as did the original population, then it would follow that selection could slide successive generations along in the direction of selection.

Darwin himself was extraordinarily careful, however, in the statements he made in this connection and it is rather by implication than by actual reference that one can ascribe this meaning to his views. His contemporaries and many of his followers, however, appear to have accepted this sliding scale interpretation as the cardinal doctrine of evolution. If this is doubted or my statement is challenged then one must explain why de Vries' mutation theory met with so little enthusiasm amongst the older group of zoölogists and botanists; and one must explain why Johannsen's splendid work met with such bitter opposition from the English school—the biometricians—who amongst the post-Darwinian school are assumed to be the lineal descendants of Darwin.

And in this connection we should not forget that just this sort of process was supposed to take place in the inheritance of use and disuse. What is gained in one generation forms the basis for further gains in the next generation. Now, Darwin not only believed that acquired characters are inherited but turned more and more to this explanation in his later writings. Let us, however, not make too much of the matter; for it is much less important to find out whether Darwin's ideas were vague, than it is to make sure that our own ideas are clear.

If I have made several statements here that appear dogmatic let me now attempt to justify them, or at least give the evidence which seems to me to make them probable.

The work of the Danish botanist, Johannsen, has given us the most carefully analyzed case of selection that has ever been obtained. There are, moreover, special reasons why the material that he used is better suited to give definite information than any other so far studied. Johannsen worked with the common bean, weighing the seeds or else measuring them. These beans if taken from many plants at random give the typical curve of probability (fig. 74). The plant multiplies by self-fertilization. Taking advantage of this fact Johannsen kept the seeds of each plant separate from the others, and raised from them a new generation. When curves were made from these new groups it was found that some of them had different modes from that of the original general population (fig. 81 A-E, bottom group). They are shown in the upper groups (A, B, C, D, E). But do not understand me to say that the offspring of each bean gave a different mode.

On the contrary, some of the lines would be the same.

The result means that the general population is made up of definite kinds of individuals that may have been sorted out.

That his conclusion is correct is shown by rearing a new generation from any plant or indeed from several plants of any one of these lines. Each line repeats the same modal class. There is no further breaking up into groups. Within the line it does not matter at all whether one chooses a big bean or a little one—they will give the same result. In a word, the germ plasm in each of these lines is pure, or homozygous, as we say. The differences that we find between the weights (or sizes) of the individual beans are due to external conditions to which they have been subjected.

In a word, Johannsen's work shows that the frequency distribution of a pure line is due to factors that are extrinsic to the germ plasm. It does not matter then which individuals in a pure line are used to breed from, for they all carry the same germ plasm.

We can now understand more clearly how selection acting on a general population brings about results in the direction of selection.

An individual is picked out from the population in order to get a particular kind of germ plasm. Although the different classes of individuals may overlap, so that one can not always judge an individual from its appearance, nevertheless on the whole chance favors the picking out of the kind of germ plasm sought.

In species with separate sexes there is the further difficulty that two individuals must be chosen for each mating, and superficial examination of them does not insure that they belong to the same group—their germ plasm cannot be inspected. Hence selection of biparental forms is a precarious process, now going forward, now backwards, now standing still. In time, however, the process forward is almost certain to take place if the selection is from a heterogeneous population. Johannsen's work was simplified because he started with pure lines. In fact, had he not done so his work would not have been essentially different from that of any selection experiment of a pure race of animals or plants. Whether Johannsen realized the importance of the condition or not is uncertain—curiously he laid no emphasis on it in the first edition of his "Elemente der exakten Erblichkeitslehre".

It has since been pointed out by Jennings and by Pearl that a race that reproduces by self-fertilization as does this bean, automatically becomes pure in all of the factors that make up its germ plasm. Since self-fertilization is the normal process in this bean the purity of the germ plasm already existed when Johannsen began to experiment.

If then selection does not bring about transgressive variation in a general population, how can selection produce anything new? If it can not produce anything new, is there any other way in which selection becomes an agent in evolution?

We can get some light on this question if we turn to what man has done with his domesticated animals and plants. Through selection, i.e., artificial selection, man has undoubtedly brought about changes as remarkable as any shown by wild animals and plants. We know, moreover, a good deal about how these changes have been wrought.

(1) By crossing different wild species or by crossing wild with races already domesticated new combinations have been made. Parts of one individual have been combined with parts of others, creating new combinations. It is possible even that characters that are entirely new may be produced by the interaction of factors brought into recombination.

(2) New characters appear from time to time in domesticated and in wild species. These, like the mutants in Drosophila, are fully equipped at the start. Since they breed true and follow Mendel's laws it is possible to combine them with characters of the wild type or with those of other mutant races.

Amongst the new mutant factors there may be some whose chief effect is on the character that the breeder is already selecting. Such a modification will be likely to attract attention. Superficially it may appear that the factor for the original character has varied, while the truth may be that another factor has appeared that has modified a character already present. In fact, many or all Mendelian factors that affect the same organ may be said to be modifiers of each other's effects. Thus the factor for vermilion causes the eye to be one color, and the factor for eosin another color, while eosin vermilion is different from both. Eosin may be said to be a modifier of vermilion or vermilion of eosin. In general, however, it is convenient to use the term "modifier" for cases in which the factor causes a detectable change in a character already present or conspicuous.

One of the most interesting, and at the same time most treacherous, kinds of modifying factors is that which produces an effect only when some other factor is present. Thus Bridges has shown that there is a factor called "cream" that does not affect the red color of the eye of the wild fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which produces no effect on red makes eosin entirely white. Since cream or whiting may be carried by red eyed flies without their presence being seen until eosin is used, the experimenter must be continually on the lookout for such factors which may lead to erroneous conclusions unless detected. As yet breeders have not realized the important rôle that modifiers have played in their results, but there are indications at least that the heaping up of modifying factors has been one of the ways in which highly specialized domesticated animals have been produced. Selection has accomplished this result not by changing factors, but by picking up modifying factors. The demonstration of the presence of these factors has already been made in some cases. Their study promises to be one of the most instructive fields for further work bearing on the selection hypothesis.

In addition to these well recognized methods by which artificial selection has produced new things we come now to a question that is the very crux of the selection theory today. Our whole conception of selection turns on the answer that we give to this matter and if I appear insistent and go into some detail it is because I think that the matter is worth very careful consideration.

As we have seen, the variation that we find from individual to individual is due in part to the environment; this can generally be demonstrated. Other differences in an ordinary population are recognized as due to different genetic (hereditary) combinations. No one will dispute this statement. But is all the variability accounted for in these two ways? May not a factor itself fluctuate? Is it not a priori probable that factors do fluctuate? Why, in a word, should we regard factors as inviolate when we see that everything else in organisms is more or less in amount? I do not know of any a priori reason why a factor may not fluctuate, unless it is, as I like to think, a chemical molecule. We are, however, dealing here not with generalities but with evidence, and there are three known methods by means of which it has been shown that variability, other than environmental or recombinational, is not due to variability in a factor, nor to various "potencies" possessed by the same factors.

(1) By making the stock uniform for all of its factors—chief factors and modifiers alike. Any change in such a stock produced by selection would then be due to a change in one or more of the factors themselves. Johannsen's experiment is an example of this sort.

(2) The second method is one that is capable of demonstrating that the effects of selection are actually due to modifiers. It has been worked out in our laboratory, chiefly by Muller, and used in a particular case to demonstrate that selection produced its effect by isolating modifying factors. For example, a mutant type called truncate appeared, characterized by shorter wings, usually square at the end, (fig. 83a). The wings varied from those of normal length to wings much shorter (fig. 83b). For three years the mutant stock was bred from individuals having the shorter wings until at last a stock was obtained in which some of the individuals had wings much shorter than the body. By means of linkage experiments it was shown that at least three factors were present that modified the wings. These were isolated by means of their linkage relations, and their mutual influence on the production of truncate wings was shown.

An experiment of this kind can only be carried out in a case where the groups of linked gens are known. At present Drosophila is the only animal (or plant) sufficiently well known to make this test possible, but this does not prove that the method is of no value. On the contrary it shows that any claim that factors can themselves be changed can have no finality until the claim can be tested out by means of the linkage test. For instance, bar eye (fig. 31) arose as a mutation. All our stock has descended from a single original mutant. But Zeleny has shown that selection within our stock will make the bar eye narrower or broader according to the direction of selection. It remains to be shown in this case how selection has produced its effects, and this can be done by utilizing the same process that was used in the case of truncate.

Another mutant stock called beaded (fig. 84), has been bred for five years and selected for wings showing more beading. In extreme cases the wings have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows great variability. It is probable here as Dexter has shown, that a number of mutant factors that act as modifiers have been picked up in the course of the selection, and when it is recalled that during those five years over 125 new characters have appeared elsewhere it does not seem improbable that factors also have appeared that modify the wings of this stock.

(3) The third method is one that has been developed principally by East for plants; also by MacDowell for rabbits and flies. The method does not claim to prove that modifiers are present, but it shows why certain results are in harmony with that expectation and can not be accounted for on the basis that a factor has changed. Let me give an example. When a Belgian hare with large body was crossed to a common rabbit with a small body the hybrid was intermediate in size. When the hybrid was crossed back to the smaller type it produced rabbits of various sizes in apparently a continuous series. MacDowell made measurements of the range of variability in the first and in the second generations.

Classification in relation to parents based on skull lengths and ulna lengths, to show the relative variability of two measurements and of the first generation (F$1$) and the back cross (B. C.)

same table continued

He found that the variability was smaller in the first generation than in the second generation (back cross). This is what is expected if several factor-differences were involved, because the hybrids of the first generation are expected to be more uniform in factorial composition than are those in the second generation which are produced by recombination of the factors introduced through their grandparents. Excellent illustrations of the same kinds of results have been found in Indian corn. As shown in figure 85 the length of the cob in F$1$ is intermediate between the parent types while in F$1$ the range is wider and both of the original types are recovered. East states that similar relations have been found for 18 characters in corn. Emerson has recently furnished further illustrations of the same relations in the length of stalks in beans.

A similar case is shown by a cross between fantail and common pigeons (fig. 86). The latter have twelve feathers in the tail, while the selected race from which the fantails came had between 28 and 38 feathers in the tail. The F$1$ offspring (forty-one individuals) showed (fig. 87) between 12 and 20 tail feathers, while in F$1$ the numbers varied between 12 and 25. Here one of the grand-parental types reappears in large numbers, while the extreme of the other grand-parental type did not reappear (in the counts obtained), although the F$1$ number would probably overlap the lower limits of the race of fantail grandparents had not a selected (surviving) lot been taken for the figures given in the table.

The preceding account attempts to point out how I should prefer to interpret the problem of selection in the light of the most recent work on breeding. But I would give a very incomplete account of the whole situation if I neglected to include some important work which has led some of my fellow-workers to a very different conclusion.

Castle in particular is the champion of a view based on his results with hooded rats. Starting with individuals which have a narrow black stripe down the back he selected for a narrower stripe in one direction and for a broader stripe in the other. As the diagram shows (fig. 88) Castle has succeeded in producing in one direction a race in which the dorsal stripe has disappeared and in the other direction a race in which the black has extended over the back and sides, leaving only a white mark on the belly. Neither of these extremes occurs, he believes, in the ordinary hooded race of domesticated rats. In other words no matter how many of them came under observation the extreme types of his experiment would not be found.

Castle claims that the factor for hoodedness must be a single Mendelian unit, because if hooded rats are crossed to wild gray rats with uniform coat and their offspring are inbred there are produced in F$2$ three uniform rats to one hooded rat. Castle advances the hypothesis that factors—by which he means Mendelian factors—may themselves vary in much the same way as do the characters that they stand for. He argues, in so many words, that since we judge a factor by the kind of character it produces, when the character varies the factor that stands for it may have changed.

As early as 1903 Cuénot had carried out experiments with spotted mice similar to those of Castle with rats. Cuénot found that spotted crossed to uniform coat color gave in F$1$ a ratio of three uniform to one spotted, yet selection of those spotted mice with more white in their coat produced mice in successive generations that had more and more white. Conversely Cuénot showed that selection of those spotted mice that had more color in their coat produced mice with more and more color and less white. Cuénot does not however bring up in this connection the question as to how selection in these spotted mice brings about its results.

Without attempting to discuss these results at the length that they deserve let me briefly state why I think Castle's evidence fails to establish his conclusion.

In the first place one of the premises may be wrong. The three to one ratio in F$2$ by no means proves that all conditions of hoodedness are due to one factor. The result shows at most that one factor that gives the hooded types is a simple Mendelian factor. The changes in this type may be caused by modifying factors that can show an effect only when hoodedness is itself present. That this is not an imaginary objection but a real one is shown by an experiment that Castle himself made which furnishes the ground for the second objection.

Second. If the factor has really changed its potency, then if a very dark individual from one end of the series is crossed to a wild rat and the second generation raised we should expect that the hooded F$1$ rats would all be dark like their dark grandparent. When Castle made this test he found that there were many grades of hooded rats in the F$2$ progeny. They were darker, it is true, as a group than were the original hooded group at the beginning of the selection experiment, but they gave many intermediate grades. Castle attempts to explain this by the assumption that the factor made pure by selection became contaminated by its normal allelomorph in the F$2$ parent, but not only does this assumption appear to beg the whole question, but it is in flat contradiction with what we have observed in hundreds of Mendelian cases where no evidence for such a contamination exists.

Later Castle crossed some of the extracted rats of average grade (3.01) from the plus series to the same wild race and got F$1$ hooded rats from this cross. These F$1$ hooded rats did not further approach the ordinary range but were nearer the extreme selected plus hooded rats (3.33) than were the F$1$'s extracted from the first cross (2.59). Castle concludes from this that multiple factors can not account for the result. As a matter of fact, Castle's evidence as published does not establish his conclusion because the wild rats used in the second experiment may have carried plus modifiers. This could only be determined by suitable tests which Castle does not furnish. This is the crucial point, without which the evidence carries no conviction.

Furthermore, from Castle's point of view, these latest results would seem to increase the difficulty of interpretation of his first F$2$ extracted cross, and it is now the first result that calls for explanation if one accepts his later conclusion.

These and other objections that might be taken up show, I think, that Castle's experiment with hooded rats fails entirely to establish his contention of change in potency of the germ or of contamination of factors, while on the contrary they are in entire accord with the view that he is dealing with a case of modifying factors.

Equally important are the results that Jennings has obtained with certain protozoa. Paramecium multiplies by dividing across in the middle, each half replacing its lacking part. Both the small nucleus (micronucleus) and the large nucleus (macronucleus) divide at each division of the body. Jennings found that while individuals descended from a single paramecium vary in size (fig. 89), yet the population from a large individual is the same as the population derived from a small individual. In other words, selection produces no result and the probable explanation is, of course, that the different sizes of individuals are due to the environment, while the constancy of the type is genetic. Jennings found a number of races of paramecium of different sizes living under natural conditions. The largest individual of a small race might overlap the smallest individual of other larger races (fig. 89); nevertheless each kind reproduced its particular race. The results are like those of Johannsen in a general way, but differ in that reproduction takes place in paramecium by direct division instead of through self-fertilization as in beans, and also in that the paramecia were probably not homozygous. Since, however, so far as known no "reduction" takes place in paramecium at each division, the genetic composition of parent and offspring should be the same. Whether pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in paramecium at intervals involves a redistribution of the hereditary factors is not clear. Jennings's evidence seems incompatible with such a view.

More recently one of Jennings's students, Middleton, has made a careful series of selection experiments with Stylonychia (fig. 90) in which he selected for lines showing more rapid or slower rates of division. His observations seem to show that his selection separated two such lines that came from the same original stock. The rapidity of the effects of selection seems to preclude the explanation that pseudo-parthenogenesis has complicated the results. Nevertheless, the results are of such a kind as to suggest that they were due to selection of vegetative (somatic) differences and that no genetic change of factors was involved, for his conclusion that the rapidity with which the effects gained by long selection might be suddenly reversed when selection was reversed is hardly consistent with an interpretation of the results based on changes in the "potencies" of the factors present.

Equally striking are the interesting experiments that Jennings has recently carried out with Difflugia (fig. 91). This protozoon secretes a shell about itself which has a characteristic shape, and often carries spines. The opening at one end of the shell through which the protoplasm protrudes to make the pseudopodia is surrounded by a rim having a characteristic pattern. The protoplasm contains several nuclei and in addition there is scattered material or particles called chromidia that are supposed to be chromatic in nature and related to the material of the nuclei, possibly by direct interchange.

When Difflugia divides, part of the protoplasm protrudes from the opening and a new shell is secreted about this mass which becomes a daughter individual. The behavior of the nucleus and of the chromidia at this time is obscure, but there is some evidence that their materials may be irregularly distributed between parent and offspring. If this is correct, and if in the protozoa the chromatin has the same influence that it seems to have in higher animals, the mode of reproduction in Difflugia would be expected to give little more than random sampling of the germ plasm.

Jennings was able by means of selection to get from the descendants of one original individual a number of different types that themselves bred true, except in so far as selection could affect another change in them. In this connection it is interesting to note that Leidy has published figures of Difflugia (fig. 92) that show that a great many "types" exist. If through sexual union (a process that occurs in Difflugia) the germ plasm (chromatin) of these wild types has in times past been recombined, then selection would be expected to separate certain types again, if, at division, irregular sampling of the germ plasm takes place. Until these points are settled the bearing of these important experiments of Jennings on the general problem of selection is uncertain.

The question still remains: Does selection play any rôle in evolution, and, if so, in what sense? Does the elimination of the unfit influence the course of evolution, except in the negative sense of leaving more room for the fit? There is something further to be said in this connection, although opinions may differ as to whether the following interpretation of the term "natural selection" is the only possible one.

If through a mutation a character appears that is neither advantageous nor disadvantageous, but indifferent, the chance that it may become established in the race is extremely small, although by good luck such a thing may occur rarely. It makes no difference whether the character in question is a dominant or a recessive one, the chance of its becoming established is exactly the same. If through a mutation a character appears that has an injurious effect, however slight this may be, it has practically no chance of becoming established.

If through a mutation a character appears that has a beneficial influence on the individual, the chance that the individual will survive is increased, not only for itself, but for all of its descendants that come to inherit this character. It is this increase in the number of individuals possessing a particular character, that might have an influence on the course of evolution. This gives a better chance for improvement by several successive steps; but not because the species is more likely to mutate again in the same direction. An imaginary example will illustrate how this happens: When elephants had trunks less than a foot long, the chance of getting trunks more than one foot long was in proportion to the length of trunks already present and to the number of individuals; but increment in trunk length is no more likely to occur from an animal having a trunk more than one foot long than from an animal with a shorter trunk.

The case is analogous to tossing pennies. At any stage in the game the chance of accumulating a hundred heads is in proportion to the number of heads already obtained, and to the number of throws still to be made. But the number of heads obtained has no influence on the number of heads that will appear in the next throw.

Owing then to this property of the germ plasm to duplicate itself in a large number of samples not only is an opportunity furnished to an advantageous variation to become extensively multiplied, but the presence of a large number of individuals of a given sort prejudices the probable future result.

The question may be raised as to whether it is desirable to call selection a creative process. There are so many supernatural and mystical implications that hang around the term creative that one can not be too careful in stating in what sense the term is to be used. If by creative is meant that something is made out of nothing, then of course there is no need for the scientist to try to answer such a question. But if by a creative process is meant that something is made out of something else, then there are two alternatives to be reckoned with.

First, if it were true that selection of an individual of a certain kind determines that new variations in the same direction occur as a consequence of the selection, then selection would certainly be creative. How this could occur might be quite unintelligible, but of course it might be claimed that the point is not whether we can explain how creation takes place, but whether we can get verifiable evidence that such a kind of thing happens. This possibility is disposed of by the fact that there is no evidence that selection determines the direction in which variation occurs.

Second, if you mean by a creative process that by picking out a certain kind of individual and multiplying its numbers a better chance is furnished that a certain end result will be obtained, such a process may be said to be creative. This is, I think, the proper use of the term creative in a mechanistic sense.

In reviewing the evidence relating to selection I have tried to handle the problem as objectively as I could.

The evidence shows clearly that the characters of wild animals and plants, as well as those of domesticated races, are inherited both in the wild and in the domesticated forms according to Mendel's Law.

The causes of the mutations that give rise to new characters we do not know, although we have no reason for supposing that they are due to other than natural processes.

Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism. Natural selection as here defined means both the increase in the number of individuals that results after a beneficial mutation has occurred (owing to the ability of living matter to propagate) and also that this preponderance of certain kinds of individuals in a population makes some further results more probable than others. More than this, natural selection can not mean, if factors are fixed and are not changed by selection.


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 filling blanks the first one proposed, or nominated, is voted on first, except where, from the nature of the case, another order is preferable, and then that order is adopted as explained above.

Nominations are treated like filling blanks; any number may be pending at the same time, not as amendments of each other, but as independent propositions to be voted on in the order in which they were made until one receives a majority vote. [See 66.]

34. To Postpone Indefinitely takes precedence of nothing except the main motion to which it is applied, and yields to all privileged [14], incidental [13], and other subsidiary [12] motions. It cannot be amended or have any other subsidiary motion applied to it except the previous question and motions limiting or extending the limits of debate. It is debatable and opens the main question to debate. It can be applied to nothing but main questions, which include questions of privilege and orders of the day after they are before the assembly for consideration. An affirmative vote on it may be reconsidered, but not a negative vote. If lost it cannot be renewed. It is simply a motion to reject the main question. If a main motion is referred to a committee while to postpone indefinitely is pending, the latter motion is ignored and does not go to the committee.

The Object of this motion is not to postpone,


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 but to reject, the main motion without incurring the risk of a direct vote on it, and it is made only by the enemies of the main motion when they are in doubt as to their being in the majority.

The Effect of making this motion is to enable members who have exhausted their right of debate on the main question, to speak again, as, technically, the question before the assembly is different, while, as far as the subject of discussion is concerned, there is no difference caused by changing the question from adopting to rejecting the measure, because the merits of the main question are open to debate in either case. If adopted, its effect is to suppress the main motion for that session, unless the vote is reconsidered. As this motion does not suppress the debate on the main question, its only useful effect is to give the opponents of the pending measure a chance of killing the main motion without risking its adoption in case of failure. For, if they carry the indefinite postponement, the main question is suppressed for the session; if they fail, they still have a vote on the main question, and, having learned their strength by the vote taken, they can form an opinion of the advisability of continuing the struggle.


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35. To Take from the Table takes precedence of no pending question, but has the right of way in preference to main motions if made during the session in which it was laid on the table while no question is actually pending, and at a time when business of this class, or unfinished business, or new business, is in order; and also during the next session in societies having regular business meetings as frequently as quarterly. It yields to privileged [14] and incidental [13] motions, but not to subsidiary [12] ones. It is undebatable, and no subsidiary motion can be applied to it. It is not in order unless some business has been transacted since the question was laid on the table, nor can it be renewed until some business has been transacted since it was lost. The motion to take from the table cannot be reconsidered, as it can be renewed repeatedly if lost, and, if carried, the question can be again laid on the table after progress in debate or business.

In ordinary deliberative assemblies, a question is supposed to be laid on the table only temporarily with the expectation of its consideration being resumed after the disposal of


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 Stewart, Mark John, M.P. Wigtown burghs 1874–80, lieut.-col. Ayrshire and Galloway artillery volunteers since 1879, M.A., Christ Church, Oxon, 1860, from Winchester Coll., a member of the Northern circuit, J.P., D.L. co. Kirkcudbright, a student of Lincoln's Inn 17 Nov., 1856 (then aged 22), went to the Inner Temple 15 and called to the bar there 27 Jan., 1862 (eldest son of Mark Sprot Stewart, Esq., of Southwick, co. Kirkcudbright, N.B.); born, 1834; married 26 July, 1866, Marianne Susanna, only child of late John Orde Ommaney, Esq., and has issue (see Foster's Scots M.P.'s).
 * Ardwell, Stranraer, Wigtown, N.B.; Southwick, Dumfries, N.B.; Carlton Club.

Stewart, Patrick, a student of the Middle Temple 24 Jan., 1844, called to the bar 29 Jan., 1847 (eldest son of Robert Stewart, Esq., of London, dec.); born, ; married 15 April, 1856, Frances Henrietta, dau. of late Richard Price, Esq., of South Lambeth; she died 6 Jan., 1864 (see Foster's Baronetage).

Stewart, William Henry, a student of the Middle Temple 11 Jan., 1844, called to the bar 29 Jan., 1847 (youngest son of Robert Stewart, Esq., of Stonehouse, Devon, dec.); born,.

Stewart, William John, B.A., Exeter Coll., Oxon, 1872, a member of the Northern circuit, a student of the Inner Temple 18 Nov., 1869 (then aged 20), called to the bar 13 June, 1877 (only son of William Goldfinch Stewart, of Liverpool, assistant collector of customs); born, 1849; married 24 Aug., 1880, Catherine Johanna, 4th dau. of P. F. Garnett, Esq., J.P., of Liverpool.
 * 30, North John Street, Liverpool.

Steyn, Martinus Theunis, a student of the Inner Temple 15 Jan., 1880 (then aged 22), called to the bar 17 Nov., 1882 (3rd son of Martinus Steyn, of Bloemfontein, Orange Free State, member of Executive Council there); born, 1858.
 * Bloemfontein, Orange Free State, South Africa.

Stickland, George John, B.A., Trin. Hall, Camb., 1840, a student of the Middle Temple 5 July, 1843, called to the bar 26 May, 1848 (only son of George Stickland, Esq., of Dorchester, dec.); born,.
 * Westbury, Sherborne, Dorset.

Stigand, William, B.A., St. John's Coll., Camb., 1851, nominated vice-consul Boulogne 1873, altered the spelling of his name, a student of Lincoln's Inn 9 June, 1846 (then aged 21), called to the bar 7 June, 1852 (only son of William Stigant, of H.M.'s dockyard, Devonport); born, 1825.

Stillwell, Edward George Ellis, a student of the Middle Temple 19 Jan., 1882, called to the bar 26 Jan., 1885 (eldest son of Edward Stillwell, late of the War Office, dec.); born,.
 * Eastcote, Priory Road, N.W.

Stines, Jacob Cohen, matric. London Univ. 1862, practises before the supreme court of Jamaica, a student of the Middle Temple 3 Nov., 1862, called to the bar 17 Nov., 1866 (eldest son of Joseph Cohen Stines, of Kingston, Jamaica, merchant); born,.
 * Kingston, Jamaica.

Stirling, Archibald William, B.C.L., Lincoln Coll., Oxon, 1879, a member of the Western circuit, practises before the supreme court of South Australia, a student of the Inner Temple 12 May, 1876 (then aged 18), called to the bar 26 Jan., 1880 (only son of late Charles Stirling, of Ashleigh House, Taunton, Somerset, and of Adelaide, South Australia); born, 1858.
 * Adelaide, South Australia.

Stirling, Arthur Frederick Gresley, B.A., Magdalen Coll., Oxon, 1870, a member of the Western circuit, a student of the Inner Temple 15 April, 1868 (then aged 21), called to the bar 30 April, 1872 (2nd son of late Charles Stirling, of Buckeridge, Devon); born, 1847.
 * 1, Chester Place, Regent's Park, N.W.

Stirling, James, M.A., Trin. Coll., Camb., 1863, junior equity counsel to the treasury 1881, member of the Bar Committee since Dec., 1883, formerly reporter for Incorporated Council, a student of Lincoln's Inn 23 Jan., 1860 (then aged 23), called to the bar 17 Nov., 1862 (eldest son of Rev. James Stirling, of Aberdeen); born 3 May, 1836; married 12 May, 1868, Aby, eldest dau. of John Thomson, Esq., of Shalford, Surrey.
 * Room 545, Royal Courts of Justice, W.C.; 61, Carey Street, W.C.

Stirling, John Lancelot, B.A., LL.B., Trin. Coll., Camb., 1871, a student of the Inner Temple 2 Nov., 1867 (then aged 17), called to the bar 18 Nov., 1872 (2nd son of Edward Stirling, Esq., of London); born, 1850.
 * Late of 34, Queen's Gardens, Hyde Park, W.

Stock, Edward Wood, LL.B., St. John's Coll., Camb., 1863, a student of Lincoln's Inn 20 April, 1849 (then aged 17), called to the bar 30 April, 1857 (eldest son of Edward Stock, Esq., of Poplar, Middlesex); born 14 Jan., 1832; married 1st, 29 Dec., 1864, Barbara Forbes, eldest dau. of James Milne Innes, of Balveny and Edengight, co. Banff); she died 24 March, 1872; he married 2ndly, 23 Dec., 1882, Lillies Georgiana, 6th dau. of late Thomas R. Leadam, Esq., M.D., of Mortimer, Berks.
 * 8, Essex Villas, Kensington, W.; 1, New Square, Lincoln's Inn, W.C.

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 the interrupting question, or at a more convenient season.* As soon as the question that was introduced when the first question was laid on the table, is disposed of, any one may move to take this first question from the table. When he rises to make the motion, if the chair recognizes some one else as having first risen, he should at once say that he rises to move to take a question from the table. The chair then assigns him the floor if the other member has risen to make a main motion. If the new main motion has been stated by the chair before he claims the floor, he must wait until that question is disposed of before his motion will be in order. When taken up, the question with everything adhering to it is before the assembly exactly as when it was laid on the table. Thus, if a resolution has amendments and a motion to commit pending at the time it was laid on the table, when it is taken from the table the question is first on the motion to commit. If a motion to postpone to a certain time is pending when the question is laid on the table, and it is taken from the table after that time, then the motion to postpone is ignored when the question is taken up. If the question is taken up on the day it was laid


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 on the table, members who have exhausted their right of debate cannot again speak on the question. But if taken up on another day, no notice is taken of speeches previously made. The previous question is not exhausted if the question upon which it was ordered is taken from the table at the same session, even though it is on another day. 36. Reconsider.* This motion is peculiar in that the making of the motion has a higher rank than its consideration, and for a certain time prevents anything being done as the result of the vote it is proposed to reconsider. It can be made only on the day the vote to be reconsidered was taken, or on the next succeeding day, a legal holiday or a recess not being counted as a day. It must be made by one who voted with the prevailing side. Any member may second it. It can be made while any other question is pending, even if another member has the floor, or after it has been voted to adjourn, provided the chair has