text: 1922 Encyclopædia Britannica/Johannesburg

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text: 1922 Encyclopædia Britannica/John, Augustus Edwyn


text: 1922 Encyclopædia Britannica/John, Griffith


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Page:The new British province of South Australia.djvu/41

22 of Australia, on a sealing voyage, in 1819. He remained at Kangaroo Island seven months, and has given a full report of his residence there, which is very valuable as being the observation of a practical Australian agriculturist, as well as the work of an experienced navigator. His report is copied at length, and it is well to state, that he submitted to a long examination on the subject before a Committee appointed to investigate the natural circumstances of the locality. The verbal evidence he then gave tended completely to corroborate his written statement.

Richard Wootton and Frederick Hamborg, the one a steward, and the other a mate, of Sealing Vessels, remained on this coast a short time, and have given some account of their voyage. The evidence of the first of these is not very important; but Hamborg states that he entered Spalding Cove, and there found two streams of water. His evidence also corroborates the account of the French navigators, as to the eligibility of Port Lincoln for the reception of a European Colony.

Captain Goold, late Master of his Majesty's Ship Dryad, and Commander of several Merchantmen, a most intelligent man, performed two voyages from Sydney to this coast in 1827 and 1828. The object of the voyage was the seal fishery. His report of the country, generally, is very important, and its value is enhanced by the fact of his having lived some time in Australia, and thus being enabled to compare the different places he visited. Ample use has been made, in the paper which follows, of his statement.

Last in order of time, but perhaps first in importance, is the account of Captain Sturt. This enterprising gentleman traced a river through an unknown country, and amidst great danger and privations, to its joining the sea at Encounter Bay. The account of his voyage, recently published by Smith and Elder, will be found most interesting, and to this we must refer all those who


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Island Life/X

CHAPTER X

THE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF ANIMALS AND PLANTS

 * Various Estimates of Geological Time—Denudation and Deposition of Strata as a Measure of Time—How to Estimate the Thickness of the Sedimentary Rocks—How to Estimate the Average Rate of Deposition of the Sedimentary Rocks—The Rate of Geological Change Probably greater in very Remote Times—Value of the Preceding Estimate of Geological Time—Organic Modification Dependent on Change of Conditions—Geographical Mutations as a Motive Power in bringing about Organic Changes—Climatal Revolutions as an Agent in Producing Organic Changes—Present Condition of the Earth one of Exceptional Stability as Regards Climate—Date of last Glacial Epoch and its Bearing on the Measurement of Geological Time—Concluding Remarks.

The subjects discussed in the last three chapters introduce us to a difficulty which has hitherto been considered a very formidable one—that the maximum age of the habitable earth, as deduced from physical considerations, does not afford sufficient time either for the geological or the organic changes of which we have evidence. Geologists continually dwell on the slowness of the processes of upheaval and subsidence, of denudation of the earth's surface, and of the formation of new strata; while on the theory of development, as expounded by Mr. Darwin, the variation and modification of organic forms is also a very slow process, and has usually been considered to require an even longer series of ages than might satisfy the requirements of physical geology alone.

As an indication of the periods usually contemplated by geologists, we may refer to Sir Charles Lyell's calculation in the tenth edition of his Principles of Geology (omitted in later editions), by which he arrived at 240 millions of years as having probably elapsed since the Cambrian period—a very moderate estimate in the opinion of most geologists. This calculation was founded on the rate of modification of the species of mollusca; but much more recently Professor Haughton has arrived at nearly similar figures from a consideration of the rate of formation of rocks and their known maximum thickness, whence he deduces a maximum of 200 millions of years for the whole duration of geological time, as indicated by the series of stratified formations. But in the opinion of all our first naturalists and geologists, the period occupied in the formation of the known stratified rocks only represents a portion, and perhaps a small portion, of geological time. In the sixth edition of the Origin of Species (p. 286), Mr. Darwin says: "Consequently, if the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures." Professor Huxley, in his anniversary address to the Geological Society in 1870, adduced a number of special cases showing that, on the theory of development, almost all the higher forms of life must have existed during the Palæozoic period. Thus, from the fact that almost the whole of the Tertiary period has been required to convert the ancestral Orohippus into the true horse, he believes that, in order to have time for the much greater change of the ancestral Ungulata into the two great odd-toed and even-toed divisions (of which change there is no trace even among the earliest Eocene mammals), we should require a large portion, if not the whole, of the Mesozoic or Secondary period. Another case is furnished by the bats and whales, both of which strange modifications of the mammalian type occur perfectly developed in the Eocene formation. What countless ages back must we then go for the origin of these groups, the whales from some ancestral carnivorous animal, and the bats from the insectivora! And even then we have to seek for the common origin of carnivora, insectivora, ungulata, and marsupials at a far earlier period; so that, on the lowest estimate, we must place the origin of the mammalia very far back in Palæozoic times. Similar evidence is afforded by reptiles, of which Professor Huxley says: "If the very small differences which are observable between the crocodiles of the older Secondary formations and those of the present day furnish any sort of an approximation towards an estimate of the average rate of change among reptiles, it is almost appalling to reflect how far back in Palæozoic times we must go before we can hope to arrive at that common stock from which the crocodiles, lizards, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch, must have been derived." Professor Ramsay has expressed similar views, derived from a general study of the whole series of geological formations and their contained fossils. He says, speaking of the abundant, varied, and well-developed fauna of the Cambrian period: "In this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series. In a broad sense, compared with what must have gone before, both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description; and the climates of seas and lands were of the very same kind as those the world enjoys at the present day."

These opinions, and the facts on which they are founded, are so weighty, that we can hardly doubt that, if the time since the Cambrian epoch is correctly estimated at 200 millions of years, the date of the commencement of life on the earth cannot be much less than 500 millions; while it may not improbably have been longer, because the reaction of the organism under changes of the environment is believed to have been less active in low and simple, than in high and complex forms of life, and thus the processes of organic development may for countless ages have been excessively slow.

But according to the physicists, no such periods as are here contemplated can be granted. From a consideration of the possible sources of the heat of the sun, as well as from calculations of the period during which the earth can have been cooling to bring about the present rate of increase of temperature as we descend beneath the surface, Sir William Thomson concludes that the crust of the earth cannot have been solidified much longer than 100 million years (the maximum possible being 400 millions), and this conclusion is held by Dr. Croll and other men of eminence to be almost indisputable. It will therefore be well to consider on what data the calculations of geologists have been founded, and how far the views here set forth, as to frequent changes of climate throughout all geological time, may affect the rate of biological change.

Denudation and Deposition of Strata as a Measure of Time.—The materials of all the stratified rocks of the globe have been obtained from the dry land. Every point of the surface is exposed to the destructive influences of sun and wind, frost, snow, and rain, which break up and wear away the hardest rocks as well as the softer deposits, and by means of rivers convey the worn material to the sea. The existence of a considerable depth of soil over the greater part of the earth's surface; of vast heaps of rocky débris at the foot of every inland cliff; of enormous deposits of gravel, sand, and loam; as well as the shingle, pebbles, sand or mud, of every sea-shore, alike attest the universality of this destructive agency. It is no less clearly shown by the way in which almost every drop of running water—whether in gutter, brooklet, stream or large river—becomes discoloured after each heavy rainfall, since the matter which causes this discolouration must be derived from the surface of the country, must always pass from a higher to a lower level, and must ultimately reach the sea, unless it is first deposited in some lake, or by the overflowing of a river goes to form an alluvial plain. The universality of this subaërial denudation, both as regards space and time, renders it certain that its cumulative effects must be very great; but no attempt seems to have been made to determine the magnitude of these effects till Mr. Alfred Tylor, in 1853, pointed out that by measuring the quantity of solid matter brought down by rivers (which can be done with considerable accuracy), we may obtain the amount of lowering of the land-area, and also the rise of the ocean level, owing to the quantity of matter deposited on its floor. A few years later Dr. Croll applied the same method in more detail to an estimate of the amount by which the land is lowered in a given period; and the validity of this method has been upheld by Sir A. Geikie, Sir Charles Lyell, and all our best geologists, as affording a means of actually determining with some approach to accuracy, the time occupied by one important phase of geological change.

The quantity of matter carried away from the land by a river is greater than at first sight appears, and is more likely to be under- than over-estimated. By taking samples of water near the mouth of a river (but above the influence of the tide) at a sufficient number of points in its channel and at different depths, and repeating this daily or at other short intervals throughout the year, it is easy to determine the quantity of solid matter held in suspension and solution; and if corresponding observations determine the quantity of water that is discharged, the total amount of solid matter brought down annually may be calculated. But besides this, a considerable quantity of sand or even gravel is carried along the bottom or bed of the river, and this has rarely been estimated, so that the figures hitherto obtained are usually under the real quantities. There is also another source of error caused by the quantity of matter the river may deposit in lakes or in flooded lands during its course, for this adds to the amount of denudation performed by the river, although the matter so deposited does not come down to the sea. After a careful examination of all the best records, Sir A. Geikie arrives at the following results, as to the quantity of matter removed by seven rivers from their basins, estimated by the number of years required to lower the whole surface an average of one foot:

Here we see an intelligible relation between the character of the river basin and the amount of denudation. The Mississippi has a large portion of its basin in an arid country, and its sources are either in forest-clad plateaux or in mountains free from glaciers and with a scanty rainfall. The Danube flows through Eastern Europe where the rainfall is considerably less than in the west, while comparatively few of its tributaries rise among the loftiest Alps. The proportionate amounts of denudation being then what we might expect, and as all are probably under rather than over the truth, we may safely take the average of them all as representing an amount of denudation which, if not true for the whole land surface of the globe, will certainly be so for a very considerable proportion of it. This average is almost exactly one foot in three thousand years. The mean altitude of the several continents has been recently estimated by Mr. John Murray, to be as follows: Europe 939 feet, Asia 3,189 feet, Africa 2020 feet, North America 1,888 feet, and South America 2,078 feet. At the rate of denudation above given, it results that, were no other forces at work, Europe would be planed down to the sea-level in about two million eight hundred thousand years; while if we take a somewhat slower rate for North America, that continent might last about four or five million years. This also implies that the mean height of these continents would have been about double what it is now three million and five million years ago respectively: and as we have no reason to suppose this to have been the case, we are led to infer the constant action of that upheaving force which the presence of sedimentary formations even on the highest mountains also demonstrates.

We have already discussed the unequal rate of denudation on hills, valleys, and lowlands, in connection with the evidence of remote glacial epochs (p. 173); what we have now to consider is, what becomes of all this denuded matter, and how far the known rate of denudation affords us a measure of the rate of deposition, and thus gives us some indication of the lapse of geological time from a comparison of this rate with the observed thickness of stratified rocks on the earth's surface.

How to Estimate the Thickness of the Sedimentary Rocks.—The sedimentary rocks of which the earth's crust is mainly composed consist, according to Sir Charles Lyell's classification, of fourteen great formations, of which the most ancient is the Laurentian, and the most recent the Post-Tertiary or Pleistocene; with thirty important subdivisions, each of which again consists of a more or less considerable number of distinct beds or strata. Thus, the Silurian formation is divided into Upper and Lower Silurian, each characterized by a distinct set of fossil remains, and the Upper Silurian again consists of a large number of separate beds, such as the Wenlock Limestone, the Upper Llandovery Sandstone the Lower Llandovery Slates, &c., each usually characterised by a difference of mineral composition or mechanical structure, as well as by some peculiar fossils. These beds and formations vary greatly in extent, both above and beneath the surface, and are also of very various thicknesses in different localities. A thick bed or series of beds often thins out in a given direction, and sometimes disappears altogether, so that two beds which were respectively above and beneath it may come into contact. As an example of this thinning out, American geologists adduce the Palæozoic formations of the Appalachian Mountains, which have a total thickness of 42,000 feet, but as they are traced westward thin out till they become only 4,000 feet in total thickness. In like manner the Carboniferous grits and shales are 18,000 feet thick in Yorkshire and Lancashire, but they thin out southwards, so that in Leicestershire they are only 3,000 feet thick; and similar phenomena occur in all strata and in every part of the world. It must be observed that this thinning out has nothing to do with denudation (which acts upon the surface of a country so as to produce great irregularities of contour), but is a regular attenuation of the layers of rock, due to a deficiency of sediment in certain directions at the original formation of the deposit. Owing to this thinning out of stratified rocks, they are on the whole of far less extent than is usually supposed. When we see a geological map showing successive formations following each other in long irregular belts across the country (as is well seen in the case of the Secondary rocks of England), and a corresponding section showing each bed dipping beneath its predecessor, we are apt to imagine that beneath the uppermost bed we should find all the others following in succession like the coats of an onion. But this is far from being the case, and a remarkable proof of the narrow limitation of these formations has been recently obtained by a boring at Ware through the Chalk and Gault Clay, which latter immediately rests on the Upper Silurian Wenlock Limestone full of characteristic fossils, at a depth of only 800 feet. Here we have an enormous gap, showing that none of earlier Secondary or late Palæozoic formations extend to this part of England, unless indeed they had been all once elevated and entirely swept away by denudation.

But if we consider how such deposits are now forming, we shall find that the thinning out of the beds of each formation, and their restriction to irregular bands and patches, is exactly what we should expect. The enormous quantity of sediment continually poured into the sea by rivers, gradually subsides to the bottom as soon as the motion of the water is checked. All the heavier material must be deposited near the shore or in those areas over which it is first spread by the tides or currents of the ocean; while only the very fine mud and clay is carried out to considerable distances. Thus all stratified deposits will form most quickly near the shores, and will thin out rapidly at greater distances, little or none being formed in the depths of the great oceans. This important fact was demonstrated by the specimens of sea-bottom examined during the voyage of the Challenger, all the "shore deposits" being usually confined within a distance of 100 or 150 miles from the coast; while the "deep-sea deposits" are either purely organic, being formed of the calcareous or siliceous skeletons of globigerinæ, radiolarians, and diatomaceæ, or are clays formed of undissolved portions of these, together with the disintegrated or dissolved materials of pumice and volcanic dust, which being very light are carried by wind or by water over the widest oceans.

From the preceding considerations we shall be better able to appreciate the calculations as to the thickness of stratified deposits made by geologists. Professor Ramsay has calculated that the sedimentary rocks of Britain alone have a total maximum thickness of 72,600 feet; while Professor Haughton, from a survey of the whole world, estimates the maximum thickness of the known stratified rocks at 177,200 feet. Now these maximum thicknesses of each deposit will have been produced only where the conditions were exceptionally favourable, either in deep water near the mouths of great rivers, or in inland seas, or in places to which the drainage of extensive countries was conveyed by ocean currents; and this great thickness will necessarily be accompanied by a corresponding thinness, or complete absence of deposit, elsewhere. How far the series of rocks found in any extensive area, as Europe or North America, represents the whole series of deposits which have been made there we cannot tell; but there is no reason to think that it is a very inadequate representation of their maximum thickness, though it undoubtedly is of their extent and bulk. When we see in how many distinct localities patches of the same formation occur, it seems improbable that the whole of the deposits formed during any one period should have been destroyed, even in such an area as Europe, while it is still more improbable that they should be so destroyed over the whole world; and if any considerable portion of them is left, that portion may give a fair idea of their average, or even of their maximum, thickness. In his admirable paper on "The Mean Thickness of the Sedimentary Rocks," Dr. James Croll has dwelt on the extent of denudation in diminishing the mean thickness of the rocks that have been formed, remarking, "Whatever the present mean thickness of all the sedimentary rocks of our globe may be, it must be small in comparison to the mean thickness of all the sedimentary rocks which have been formed. This is obvious from the fact that the sedimentary rocks of one age are partly formed from the destruction of the sedimentary rocks of former ages. From the Laurentian age down to the present day the stratified rocks have been undergoing constant denudation." This is perfectly true, and yet the mean thickness of that portion of the sedimentary rocks which remains may not be very different from that of the entire mass, because denudation acts only on those rocks which are exposed on the surface of a country, and most largely on those that are upheaved; while, except in the rare case of an extensive formation being quite horizontal, and wholly exposed to the sea or to the atmosphere, denudation can have no tendency to diminish the thickness of any entire deposit. Unless, therefore, a formation is completely destroyed by denudation in every part of the world (a thing very improbable), we may have in existing rocks a not very inadequate representation of the mean thickness of all that have been formed, and even of the maximum thickness of the larger portion. This will be the more likely because it is almost certain that many rocks contemporaneously formed are counted by geologists as distinct formations, whenever they differ in lithological character or in organic remains. But we know that limestones, sandstones, and shales, are always forming at the same time; while a great difference in organic remains may arise from comparatively slight changes of geographical features, or from difference in the depth or purity of the water in which the animals lived.

How to Estimate the Average Rate of Deposition of the Sedimentary Rocks.—But if we take the estimate of Professor Haughton (177,200 feet), which, as we have seen, is probably excessive, for the maximum thickness of the sedimentary rocks of the globe of all known geological ages, can we arrive at any estimate of the rate at which they were formed? Dr. Croll has attempted to make such an estimate, but he has taken for his basis the mean thickness of the rocks, which we have no means whatever of arriving at, and which he guesses, allowing for denudation, to be equal to the maximum thickness as measured by geologists. The land-area of the globe is, according to Dr. Croll, 57,000,000 square miles, and he gives the coast-line as 116,000 miles. This, however, is, for our purpose, rather too much, as it allows for bays, inlets, and the smaller islands. An approximate measurement on a globe shows that 100,000 miles will be nearer the mark, and this has the advantage of being an easily remembered even number. The distance from the coast, to which shore-deposits usually extend, may be reckoned at about 100 or 150 miles, but by far the larger portion of the matter brought down from the land will be deposited comparatively close to the shore; that is, within twenty or thirty miles. If we suppose the portion deposited beyond thirty miles to be added to the deposits within that distance, and the whole reduced to a uniform thickness in a direction at right angles to the coast, we should probably include all areas where deposits of the maximum thickness are forming at the present time, along with a large but unknown proportion of surface where the deposits were far below the maximum thickness. This follows, if we consider that deposit must go on very unequally along different parts of a coast, owing to the distance from each other of the mouths of great rivers and the limitations of ocean currents; and because, compared with the areas over which a thick deposit is forming annually, those where there is little or none are probably at least twice as extensive. If, therefore, we take a width of thirty miles along the whole coast-line of the globe as representing the area over which deposits are forming, corresponding to the maximum thickness as measured by geologists, we shall certainly over rather than under-estimate the possible rate of deposit.

Now a coast line of 100,000 miles with a width of 30 gives an area of 3,000,000 square miles, on which the denuded matter of the whole land-area of 57,000,000 square miles is deposited. As these two areas are as 1 to 19, it follows that deposition, as measured by maximum thickness, goes on at least nineteen times as fast as denudation—probably very much faster. But the mean rate of denudation over the whole earth is about one foot in three thousand years; therefore the rate of maximum deposition will be at least 19 feet in the same time; and as the total maximum thickness of all the stratified rocks of the globe is, according to Professor Haughton, 177,200 feet, the time required to produce this thickness of rock, at the present rate of denudation and deposition, is only 28,000,000 years.

The Rate of Geological Change Probably Greater in very Remote Times.—The opinion that denudation and deposition went on more rapidly in earlier times owing to the frequent occurrence of vast convulsions and cataclysms was strenuously opposed by Sir Charles Lyell, who so well showed that causes of the very same nature as those now in action were sufficient to account for all the phenomena presented by the rocks throughout the whole series of geological formations. But while upholding the soundness of the views of the "uniformitarians" as opposed to the "convulsionists," we must yet admit that there is reason for believing in a gradually increasing intensity of all telluric action as we go back into past time. This subject has been well treated by Mr. W. J. Sollas, who shows that, if, as all physicists maintain, the sun gave out perceptibly more heat in past ages than now, this alone would cause an increase in almost all the forces that have brought about geological phenomena. With greater heat there would be a more extensive aqueous atmosphere, and, perhaps, a greater difference between equatorial and polar temperatures; hence more violent winds, heavier rains and snows, and more powerful oceanic currents, all producing more rapid denudation. At the same time, the internal heat of the earth being greater, it would be cooling more rapidly, and thus the forces of contraction—which cause the upheaving of mountains, the eruption of volcanoes, and the subsidence of extensive areas—would be more powerful and would still further aid the process of denudation. Yet again, the earth's rotation was certainly more rapid in very remote times, and this would cause more impetuous tides and still further add to the denuding power of the ocean. It thus appears that, as we go back into the past, all the forces tending to the continued destruction and renewal of the earth's surface would be in more powerful action, and must therefore tend to reduce the time required for the deposition and upheaval of the various geological formations. It may be true, as many geologists assert, that the changes here indicated are so slow that they would produce comparatively little effect within the time occupied by the known sedimentary rocks, yet, whatever effect they did produce would certainly be in the direction here indicated, and as several causes are acting together, their combined effects may have been by no means unimportant. It must also be remembered that such an increase of the primary forces on which all geologic change depends would act with great effect in still further intensifying those alternations of cold and warm periods in each hemisphere, or, more frequently, of excessive and equable seasons, which have been shown to be the result of astronomical, combined with geographical, revolutions; and this would again increase the rapidity of denudation and deposition, and thus still further reduce the time required for the production of the known sedimentary rocks. It is evident therefore that these various considerations all combine to prove that, in supposing that the rate of denudation has been on the average only what it is now, we are almost certainly over-estimating the time required to have produced the whole series of formations from the Cambrian upwards.

Value of the Preceding Estimate of Geological Time.—It is not of course supposed that the calculation here given makes any approach to accuracy, but it is believed that it does indicate the order of magnitude of the time required. We have a certain number of data, which are not guessed but the result of actual measurement; such are, the amount of solid matter carried down by rivers, the width of the belt within which this matter is mainly deposited, and the maximum thickness of the known stratified rocks. A considerable but unknown amount of denudation is effected by the waves of the ocean eating away coast lines. This was once thought to be of more importance than sub-aërial denudation, but it is now believed to be comparatively slow in its action. Whatever it may be, however, it adds to the rate of formation of new strata, and its omission from the calculation is again on the side of making the lapse of time greater rather than less than the true amount. Even if a considerable modification should be needed in some of the assumptions it has been necessary to make, the result must still show that, so far as the time required for the formation of the known stratified rocks, the hundred million years allowed by physicists is not only ample, but will permit of even more than an equal period anterior to the lowest Cambrian rocks, as demanded by Mr. Darwin—a demand supported and enforced by the arguments, taken from independent standpoints, of Professor Huxley and Professor Ramsay.

Organic Modification Dependent on Change of Conditions.—Having thus shown that the physical changes of the earth's surface may have gone on much more rapidly and occupied much less time than has generally been supposed, we have now to inquire whether there are any considerations which lead to the conclusion that organic changes may have gone on with corresponding rapidity.

There is no part of the theory of natural selection which is more clear and satisfactory than that which connects changes of specific forms with changes of external conditions or environment. If the external world remains for a moderate period unchanged, the organic world soon reaches a state of equilibrium through the struggle for existence; each species occupies its place in nature, and there is then no inherent tendency to change. But almost any change whatever in the external world disturbs this equilibrium, and may set in motion a whole series of organic revolutions before it is restored. A change of climate in any direction will be sure to injure some and benefit other species. The one will consequently diminish, the other increase in number; and the former may even become extinct. But the extinction of a species will certainly affect other species which it either preyed upon, or competed with, or served for food; while the increase of any one animal may soon lead to the extinction of some other to which it was inimical. These changes will in their turn bring other changes; and before an equilibrium is again established, the proportions, ranges, and numbers, of the species inhabiting the country may be materially altered. The complex manner in which animals are related to each other is well exhibited by the importance of insects, which in many parts of the world limit the numbers or determine the very existence of some of the higher animals. Mr. Darwin says:—"Perhaps Paraguay offers the most curious instance of this; for here neither cattle, nor horses, nor dogs have ever run wild, though they swarm southward and northward in a wild state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of navel-frequenting flies—then cattle and horses would run wild; and this would certainly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects, and this, as we have seen in Staffordshire, the insectivorous birds, and so onwards in ever increasing circles of complexity."

Geographical changes would be still more important, and it is almost impossible to exaggerate the modifications of the organic world that might result from them. A subsidence of land separating a large island from a continent would affect the animals and plants in a variety of ways. It would at once modify the climate, and so produce a series of changes from this cause alone; but more important would be its effect by isolating small groups of individuals of many species and thus altering their relations to the rest of the organic world. Many of these would at once be exterminated, while others, being relieved from competition, might flourish and become modified into new species. Even more striking would be the effects when two continents, or any two land areas which had been long separated, were united by an upheaval of the strait which divided them. Numbers of animals would now be brought into competition for the first time. New enemies and new competitors would appear in every part of the country; and a struggle would commence which, after many fluctuations, would certainly result in the extinction of some species, the modification of others, and a considerable alteration in the proportionate numbers and the geographical distribution of almost all.

Any other changes which led to the intermingling of species whose ranges were usually separate would produce corresponding results. Thus, increased severity of winter or summer temperature, causing southward migrations and the crowding together of the productions of distinct regions, must inevitably produce a struggle for existence, which would lead to many changes both in the characters and the distribution of animals. Slow elevations of the land would produce another set of changes, by affording an extended area in which the more dominant species might increase their numbers; and by a greater range and variety of alpine climates and mountain stations, affording room for the development of new forms of life.

Geographical Mutations as a Motive Power in Bringing about Organic Changes.—Now, if we consider the various geographical changes which, as we have seen, there is good reason to believe have ever been going on in the world, we shall find that the motive power to initiate and urge on organic changes has never been wanting. In the first place, every continent, though permanent in a general sense, has been ever subject to innumerable physical and geographical modifications. At one time the total area has increased, and at another has diminished; great plateaus have gradually risen up, and have been eaten out by denudation into mountain and valley; volcanoes have burst forth, and, after accumulating vast masses of eruptive matter, have sunk down beneath the ocean, to be covered up with sedimentary rocks, and at a subsequent period again raised above the surface; and the loci of all these grand revolutions of the earth's surface have changed their position age after age, so that each portion of every continent has again and again been sunk under the ocean waves, formed the bed of some inland sea, or risen high into plateaus and mountain ranges. How great must have been the effects of such changes on every form of organic life! And it is to such as these we may perhaps trace those great changes of the animal world which have seemed to revolutionise it, and have led us to class one geological period as the age of reptiles, another as the age of fishes, and a third as the age of mammals.

But such changes as these must necessarily have led to repeated unions and separations of the land masses of the globe, joining together continents which were before divided, and breaking up others into great islands or extensive archipelagoes. Such alterations of the means of transit would probably affect the organic world even more profoundly than the changes of area, of altitude, or of climate, since they afforded the means, at long intervals, of bringing the most diverse forms into competition, and of spreading all the great animal and vegetable types widely over the globe. But the isolation of considerable masses of land for long periods also afforded the means of preservation to many of the lower types, which thus had time to become modified into a variety of distinct forms, some of which became so well adapted to special modes of life that they have continued to exist to the present day, thus affording us examples of the life of early ages which would probably long since have become extinct had they been always subject to the competition of the more highly organised animals. As examples of such excessively archaic forms, we may mention the mud-fishes and the ganoids, confined to limited fresh-water areas; the frogs and toads, which still maintain themselves vigorously in competition with higher forms; and among mammals the Ornithorhynchus and Echidna of Australia; the whole order of Marsupials—which, out of Australia, where they are quite free from competition, only exist abundantly in South America, which was certainly long isolated from the northern continents; the Insectivora, which, though widely scattered, are generally nocturnal or subterranean in their habits; and the Lemurs, which are most abundant in Madagascar, where they have long been isolated, and almost removed from the competition of higher forms.

Climatal Revolutions as an Agent in Producing Organic Changes.—The geographical and geological changes we have been considering are probably those which have been most effective in bringing about the great features of the distribution of animals, as well as the larger movements in the development of organised beings; but it is to the alternations of warm and cold, or of uniform and excessive climates—of almost perpetual spring in arctic as well as in temperate lands, with occasional phases of cold culminating at remote intervals in glacial epochs,—that we must impute some of the more remarkable changes both in the specific characters and in the distribution of organisms. Although the geological evidence is opposed to the belief in early glacial epochs except at very remote and distant intervals, there is nothing which contradicts the occurrence of repeated changes of climate, which, though too small in amount to produce any well-marked physical or organic change, would yet be amply sufficient to keep the organic world in a constant state of movement, and which, by subjecting the whole flora and fauna of a country at comparatively short intervals to decided changes of physical conditions, would supply that stimulus and motive power which, as we have seen, is all that is necessary to keep the processes of "natural selection" in constant operation.

The frequent recurrence of periods of high and of low excentricity must certainly have produced changes of climate of considerable importance to the life of animals and plants. During periods of high excentricity with summer in perihelion, that season would be certainly very much hotter, while the winters would be longer and colder than at present; and although geographical conditions might prevent any permanent increase of snow and ice even in the extreme north, yet we cannot doubt that the whole northern hemisphere would then have a very different climate than when the changing phase of precession brought a very cool summer and a very mild winter—a perpetual spring, in fact. Now, such a change of climate would certainly be calculated to bring about a considerable change of species, both by modification and migration, without any such decided change of type either in the vegetation or the animals that we could say from their fossil remains that any change of climate had taken place. Let us suppose, for instance, that the climate of England and that of Canada were to be mutually exchanged, and that the change took five or six thousand years to bring about, it cannot be doubted that considerable modifications in the fauna and flora of both countries would be the result, although it is impossible to predict what the precise changes would be. We can safely say, however, that some species would stand the change better than others, while it is highly probable that some would be actually benefited by it, and that others would be injured. But the benefited would certainly increase, and the injured decrease, in consequence, and thus a series of changes would be initiated that might lead to most important results. Again, we are sure that some species would become modified in adaptation to the change of climate more readily than others, and these modified species would therefore increase at the expense of others not so readily modified; and hence would arise on the one hand extinction of species, and on the other the production of new forms.

But this is the very least amount of change of climate that would certainly occur every 10,500 years when there was a high excentricity, for it is impossible to doubt that a varying distance of the sun in summer from 86 to 99 millions of miles (which is what occurred during—as supposed—the Miocene period, 850,000 years ago) would produce an important difference in the summer temperature and in the actinic influence of sunshine on vegetation. For the intensity of the sun's rays would vary as the square of the distance, or nearly as 74 to 98, so that the earth would be actually receiving one-fourth less sun-heat during summer at one time than at the other. An equally high excentricity occurred 2,500,000 years back, and no doubt was often reached during still earlier epochs, while a lower but still very high excentricity has frequently prevailed, and is probably near its average value. Changes of climate, therefore, every 10,500 years, of the character above indicated and of varying intensity, have been the rule rather than the exception in past time; and these changes must have been variously modified by changing geographical conditions so as to produce climatic alterations in different directions, giving to the ancient lands either dry or wet seasons, storms or calms, equable or excessive temperatures, in a variety of combinations of which the earth perhaps affords no example under the present low phase of excentricity and consequent slight inequality of sun-heat.

Present Condition of the Earth One of Exceptional Stability as Regards Climate.—It will be seen, by a reference to the diagram at page 171, that during the last three million years the excentricity has been less than it is now on eight occasions, for short periods only, making up a total of about 280,000 years; while it has been more than it is now for many long periods, of from 300,000 to 700,000 years each, making a total of 2,720,000 years; or nearly as 10 to 1. For nearly half the entire period, or 1,400,000 years, the excentricity has been nearly double what it is now, and this is not far from its mean condition. We have no reason for supposing that this long period of three million years, for which we have tables, was in any way exceptional as regards the degree or variation of excentricity; but, on the contrary, we may pretty safely assume that its variations during this time fairly represent its average state of increase and decrease during all known geological time. But when the glacial epoch ended, 72,000 years ago, the excentricity was about double its present amount; it then rapidly decreased till, at 60,000 years back, it was very little greater than it is now, and since then it has been uniformly small. It follows that, for about 60,000 years before our time, the mutations of climate every 10,500 years have been comparatively unimportant, and that the temperate zones have enjoyed an exceptional stability of climate. During this time those powerful causes of organic change which depend on considerable changes of climate and the consequent modifications, migrations, and extinctions of species, will not have been at work; the slight changes that did occur would probably be so slow and so little marked that the various species would be able to adapt themselves to them without much disturbance; and the result would be an epoch of exceptional stability of species.

But it is from this very period of exceptional stability that we obtain our only scale for measuring the rate of organic change. It includes not only the historical period, but that of the Swiss Lake dwellings, the Danish shell-mounds, our peat-bogs, our sunken forests, and many of our superficial alluvial deposits—the whole in fact, of the iron, bronze, and neolithic ages. Even some portion of the palæolithic age, and of the more recent gravels and cave-earths may come into the same general period if they were formed when the glacial epoch was passing away. Now throughout all these ages we find no indication of change of species, and but little, comparatively, of migration. We thus get an erroneous idea of the permanence and stability of specific forms, due to the period immediately antecedent to our own being a period of exceptional permanence and stability as regards climatic and geographical conditions.

Date of Last Glacial Epoch and its Bearing on the Measurement of Geological Time.—Directly we go back from this stable period we come upon changes both in the forms and in the distribution of species; and when we pass beyond the last glacial epoch into the Pliocene period we find ourselves in a comparatively new world, surrounded by a considerable number of species altogether different from any which now exist, together with many others which, though still living, now inhabit distant regions. It seems not improbable that what is termed the Pliocene period, was really the coming on of the glacial epoch, and this is the opinion of Professor Jules Marcou. According to our views, a considerable amount of geographical change must have occurred at the change from the Miocene to the Pliocene, favouring the refrigeration of the northern hemisphere, and leading, in the way already pointed out, to the glacial epoch whenever a high degree of excentricity prevailed. As many reasons combine to make us fix the height of the glacial epoch at the period of high excentricity which occurred 200,000 years back, and as the Pliocene period was probably not of long duration, we must suppose the next great phase of very high excentricity (850,000 years ago) to fall within the Miocene epoch. Dr. Croll believes that this must have produced a glacial period, but we have shown strong reasons for believing that, in concurrence with favourable geographical conditions, it led to uninterrupted warm climates in the temperate and northern zones. This, however, did not prevent the occurrence of local glaciation wherever other conditions led to its initiation, and the most powerful of such conditions is a great extent of high land. Now we know that the Alps acquired a considerable part of their elevation during the latter part of the Miocene period, since Miocene rocks occur at an elevation of over 6,000 feet, while Eocene beds occur at nearly 10,000 feet. But since that time there has been a vast amount of denudation, so that these rocks may have been at first raised much higher than we now find them, and thus a considerable portion of the Alps may have been more elevated than they are now. This would certainly lead to an enormous accumulation of snow, which would be increased when the excentricity reached a maximum, as already fully explained, and may then have caused glaciers to descend into the adjacent sea, carrying those enormous masses of rock which are buried in the Upper Miocene of the Superga in Northern Italy. An earlier epoch of great altitude in the Alps coinciding with the very high excentricity 2,500,000 years ago, may have caused the local glaciation of the Middle Eocene period when the enormous erratics of the Flysch conglomerate were deposited in the inland seas of Northern Switzerland, the Carpathians, and the Apennines. This is quite in harmony with the indications of an uninterrupted warm climate and rich vegetation during the very same period in the adjacent low countries, just as we find at the present day in New Zealand a delightful climate and a rich vegetation of Metrosideros, fuchsias and tree-ferns on the very borders of huge glaciers, descending to within 700 feet of the sea-level. It is not pretended that these estimates of geological time have any more value than probable guesses; but it is certainly a curious coincidence that two remarkable periods of high excentricity should have occurred, at such periods and at such intervals apart, as very well accord with the comparative remoteness of the two deposits in which undoubted signs of ice-action have been found, and that both these are localised in the vicinity of mountains which are known to have acquired a considerable elevation at about the same period of time.

In the tenth edition of the Principles of Geology, Sir Charles Lyell, taking the amount of change in the species of mollusca as a guide, estimated the time elapsed since the commencement of the Miocene as one-third that of the whole Tertiary epoch, and the latter at one-fourth that of geological time since the Cambrian period. Professor Dana, on the other hand, estimates the Tertiary as only one-fifteenth of the Mesozoic and Palæozoic combined. On the estimate above given, founded on the dates of phases of high excentricity, we shall arrive at about four million years for the Tertiary epoch, and sixteen million years for the time elapsed since the Cambrian, according to Lyell, or sixty millions according to Dana. The estimate arrived at from the rate of denudation and deposition (twenty-eight million years) is nearly midway between these, and it is, at all events, satisfactory that the various measures result in figures of the same order of magnitude, which is all one can expect when discussing so difficult and exceedingly speculative a subject.

The only value of such estimates is to define our notions of geological time, and to show that the enormous periods, of hundreds of millions of years, which have sometimes been indicated by geologists, are neither necessary nor warranted by the facts at our command; while the present result places us more in harmony with the calculations of physicists, by leaving a very wide margin between geological time as defined by the fossiliferous rocks, and that far more extensive period which includes all possibility of life upon the earth.

Concluding Remarks.—In the present chapter I have endeavoured to show that, combining the measured rate of denudation with the estimated thickness and probable extent of the known series of sedimentary rocks, we may arrive at a rude estimate of the time occupied in the formation of those rocks. From another point of departure—that of the probable date of the Miocene period, as determined by the epoch of high excentricity supposed to have aided in the production of the Alpine glaciation during that period, and taking the estimate of geologists as to the proportionate amount of change in the animal world since that epoch—we obtain another estimate of the duration of geological time, which, though founded on far less secure data, agrees pretty nearly with the former estimate. The time thus arrived at is immensely less than the usual estimates of geologists, and is so far within the limits of the duration of the earth as calculated by Sir William Thomson, as to allow for the development of the lower organisms an amount of time anterior to the Cambrian period several times greater than has elapsed between that period and the present day. I have further shown that, in the continued mutations of climate produced by high excentricity and opposite phases of precession, even though these did not lead to glacial epochs, we have a motive power well calculated to produce far more rapid organic changes than have hitherto been thought possible; while in the enormous amount of specific variation (as demonstrated in an earlier chapter), we have ample material for that power to act upon, so as to keep the organic world in a state of rapid change and development proportioned to the comparatively rapid changes in the earth's surface.

We have now finished the series of preliminary studies of the biological conditions and physical changes which have affected the modification and dispersal of organisms, and have thus brought about their actual distribution on the surface of the earth. These studies will, it is believed, place us in a condition to solve most of the problems presented by the distribution of animals and plants, whenever the necessary facts, both as to their distribution and their affinities, are sufficiently well known; and we now proceed to apply the principles we have established to the interpretation of the phenomena presented by some of the more important and best known of the islands of our globe, limiting ourselves to these for reasons which have been already sufficiently explained in our preface.

82 Nature, Vol. XVIII. (July, 1878), p. 268.

83 "On the Comparative Value of certain Geological Ages considered as items of Geological Time." (Proceedings of the Royal Society, 1874, p. 334.)

84 ''Trans. Royal Society of Edinburgh'', Vol. XXIII. p. 161. Quarterly Journal of Science, 1877. (Croll on the "Probable Origin and Age of the Sun.")

85 Philosophical Magazine, April, 1853.

86 It has usually been the practice to take the amount of denudation in the Mississippi valley, or one foot in six thousand years, as a measure of the rate of denudation in Europe, from an idea apparently of being on the "safe side," and of not over-estimating the rate of change. But this appears to me a most unphilosophical mode of proceeding and unworthy of scientific inquiry. What should we think of astronomers if they always took the lowest estimates of planetary or stellar distances, instead of the mean results of observation, "in order to be on the safe side!"? As if error in one direction were any worse than error in another. Yet this is what geologists do systematically. Whenever any calculations are made involving the antiquity of man, it is those that give the lowest results that are always taken, for no reason apparently except that there was, for so long a time, a prejudice, both popular and scientific, against the great antiquity of man; and now that a means has been found of measuring the rate of denudation, they take the slowest rate instead of the mean rate, apparently only because there is now a scientific prejudice in favour of extremely slow geological change. I take the mean of the whole; and as this is almost exactly the same as the mean of the three great European rivers—the Rhone, Danube, and Po—I cannot believe that this will not be nearer the truth for Europe than taking one North American river as the standard.

87 "On the Height of the Land and the Depth of the Ocean," in the Scottish Geographical Magazine, 1888.

88 These figures are merely used to give an idea of the rate at which denudation is actually going on now; but if no elevatory forces were at work, the rate of denudation would certainly diminish as the mountains were lowered and the slope of the ground everywhere rendered flatter. This would follow not only from the diminished power of rain and rivers, but because the climate would become more uniform, the rainfall probably less, and no rocky peaks would be left to be fractured and broken up by the action of frosts. It is certain, however, that no continent has ever remained long subject to the influences of denudation alone, for, as we have seen in our sixth chapter, elevation and depression have always been going on in one part or other of the surface.

89 The following statement of the depths at which the Palæozoic formations have been reached in various localities in and round London was given by Mr. H. B. Woodward in his address to the Norwich Geological Society in 1879:—

We thus find that over a wide area, extending from London to Ware and Harwich, the whole of the formations from the Oolite to the Permian are wanting, the Cretaceous resting on the Carboniferous or older Palæozoic rocks; and the same deficiency extends across to Belgium, where the Tertiary beds are found resting on Carboniferous at a depth of less than 400 feet.

90 Geological Magazine, Vol. VIII., March, 1871.

91 Mr. C. Lloyd Morgan has well illustrated this point by comparing the generally tilted-up strata denuded on their edges, to a library in which a fire had acted on the exposed edges of the books, destroying a great mass of literature but leaving a portion of each book in its place, which portion represents the thickness but not the size of the book. (Geological Magazine, 1878, p. 161.)

92 Professor J. Young thinks it highly probable that—"the Lower Greensand is contemporaneous with part of the Chalk, so were parts of the Wealden; nay, even of the Purbeck a portion must have been forming while the Cretaceous sea was gradually deepening southward and westward." Yet these deposits are always arranged successively, and their several thicknesses added together to obtain the total thickness of the formations of the country. (See Presidential Address, Sect. C. British Association, 1876.)

93 Mr. John Murray in his more careful estimate makes it about 51½ millions.

94 As by far the larger portion of the denuded matter of the globe passes to the sea through comparatively few great rivers, the deposits must often be confined to very limited areas. Thus the denudation of the vast Mississippi basin must be almost all deposited in a limited portion of the Gulf of Mexico, that of the Nile within a small area of the Eastern Mediterranean, and that of the great rivers of China—the Hoang Ho and Yang-tse-kiang, in a small portion of the Eastern Sea. Enormous lengths of coast, like those of Western America and Eastern Africa, receive very scanty deposits; so that thirty miles in width along the whole of the coasts of the globe will probably give an area greater than that of the area of average deposit, and certainly greater than that of maximum deposit, which is the basis on which I have here made my estimates. In the case of the Mississippi, it is stated by Count Pourtales that along the plateau between the mouth of the river and the southern extremity of Florida for two hundred and fifty miles in width the bottom consists of clay with some sand and but few Rhizopods; but beyond this distance the soundings brought up either Rhizopod shells alone, or these mixed with coral sand, Nullipores, and other calcareous organisms (Dana's Manual of Geology, 2nd Ed. p. 671). It is probable, therefore, that a large proportion of the entire mass of sediment brought down by the Mississippi is deposited on the limited area above indicated.

Professor Dana further remarks: "Over interior oceanic basins as well as off a coast in quiet depths, fifteen or twenty fathoms and beyond, the deposits are mostly of fine silt, fitted for making fine argillaceous rocks, as shales or slates. When, however, the depth of the ocean falls off below a hundred fathoms, the deposition of silt in our existing oceans mostly ceases, unless in the case of a great bank along the border of a continent."

95 From the same data Professor Haughton estimates a minimum of 200 million years for the duration of geological time; but he arrives at this conclusion by supposing the products of denudation to be uniformly spread over the whole sea-bottom instead of over a narrow belt near the coasts, a supposition entirely opposed to all the known facts, and which had been shown by Dr. Croll, five years previously, to be altogether erroneous. (See Nature, Vol. XVIII., p. 268, where Professor Haughton's paper is given as read before the Royal Society.)

96 See Geological Magazine for 1877, p. 1.

97 In his reply to Sir W. Thomson, Professor Huxley assumed one foot in a thousand years as a not improbable rate of deposition. The above estimate indicates a far higher rate; and this follows from the well-ascertained fact, that the area of deposition is many times smaller than the area of denudation.

98 Dr. Croll and Sir Archibald Geikie have shown that marine denudation is very small in amount as compared with sub-aërial, since it acts only locally on the edge of the land, whereas the latter acts over every foot of the surface. Mr. W. T. Blanford argues that the difference is still greater in tropical than in temperate latitudes, and arrives at the conclusion that—"If over British India the effects of marine to those of fresh-water denudation in removing the rocks of the country be estimated at 1 to 100, I believe that the result of marine action will be greatly overstated" (Geology and Zoology of Abyssinia, p. 158, note). Now, as our estimate of the rate of sub-aërial denudation cannot pretend to any precise accuracy, we are justified in neglecting marine denudation altogether, especially as we have no method of estimating it for the whole earth with any approach to correctness.

99 Agassiz appears to have been the first to suggest that the principal epochs of life extermination were epochs of cold; and Dana thinks that two at least such epochs may be recognised, at the close of the Palæozoic and of the Cretaceous periods—to which we may add the last glacial epoch.

100 This view was, I believe, first put forth by myself in a paper read before the Geological Section of the British Association in 1869, and subsequently in an article in Nature, Vol. I. p. 454. It was also stated by Mr. S. B. J. Skertchley in his Physical System of the Universe, p. 363 (1878); but we both founded it on what I now consider the erroneous doctrine that actual glacial epochs recurred each 10,500 years during periods of high excentricity.

101 Explication d'une seconde édition de la Carte Géologique de la Terre (1875), p. 64.

title: Island Life/X


text:
Island Life/XI

CHAPTER XI

THE CLASSIFICATION OF ISLANDS

 * Importance of Islands in the Study of the Distribution of Organisms—Classification of Islands with Reference to Distribution—Continental Islands—Oceanic Islands.

In the preceding chapters, forming the first part of our work, we have discussed, more or less fully, the general features presented by animal distribution, as well as the various physical and biological changes which have been the most important agents in bringing about the present condition of the organic world.

We now proceed to apply these principles to the solution of the numerous problems presented by the distribution of animals; and in order to limit the field of our inquiry, and at the same time to deal only with such facts as may be rendered intelligible and interesting to those readers who have not much acquaintance with the details of natural history, we propose to consider only such phenomena as are presented by the islands of the globe.

Importance of Islands in the Study of the Distribution of Organisms.—Islands possess many advantages for the study of the laws and phenomena of distribution. As compared with continents they have a restricted area and definite boundaries, and in most cases their geographical and biological limits coincide. The number of species and of genera they contain is always much smaller than in the case of continents, and their peculiar species and groups are usually well defined and strictly limited in range. Again, their relations with other lands are often direct and simple, and even when more complex are far easier to comprehend than those of continents; and they exhibit besides certain influences on the forms of life and certain peculiarities in their distribution which continents do not present, and whose study offers many points of interest.

In islands we have the facts of distribution presented to us, sometimes in their simplest forms, in other cases becoming gradually more and more complex; and we are therefore able to proceed step by step in the solution of the problems they present. But as in studying these problems we have necessarily to take into account the relations of the insular and continental faunas, we also get some knowledge of the latter, and acquire besides so much command over the general principles which underlie all problems of distribution, that it is not too much to say that when we have mastered the difficulties presented by the peculiarities of island life we shall find it comparatively easy to deal with the more complex and less clearly defined problems of continental distribution.

Classification of Islands with Reference to Distribution.—Islands have had two distinct modes of origin—they have either been separated from continents of which they are but detached fragments, or they have originated in the ocean and have never formed part of a continent or any large mass of land. This difference of origin is fundamental, and leads to a most important difference in their animal inhabitants; and we may therefore first distinguish the two classes—oceanic and continental islands.

Mr. Darwin appears to have been the first writer who called attention to the number and importance, both from a geological and biological point of view, of oceanic islands. He showed that with very few exceptions all the remoter islands of the great oceans were of volcanic or coralline formation, and that none of them contained indigenous mammalia or amphibia. He also showed the connection of these two phenomena, and maintained that none of the islands so characterised had ever formed part of a continent. This was quite opposed to the opinions of the scientific men of the day, who almost all held the idea of continental extensions, and of oceanic islands being their fragments, and it was long before Mr. Darwin's views obtained general acceptance. Even now the belief still lingers; and we continually hear of old Atlantic or Pacific continents, of "Atlantis" or "Lemuria," of which hypothetical lands many existing islands, although wholly volcanic, are thought to be the remnants. We have already seen that Darwin connected the peculiar geological structure of oceanic islands with the permanence of the great oceans which contain them, and we have shown that several distinct lines of evidence all point to the same conclusion. We may therefore define oceanic islands, as follows:—Islands of volcanic or coralline formation, usually far from continents and always separated from them by very deep sea, entirely without indigenous land mammalia or amphibia, but with a fair number of birds and insects, and usually with some reptiles. This definition will exclude only two islands which have been sometimes classed as oceanic—New Zealand and the Seychelles. Rodriguez, which was once thought to be another exception, has been shown by the explorations during the Transit of Venus Expedition to be essentially volcanic, with some upraised coralline limestone.

Continental Islands.—Continental islands are always more varied in their geological formation, containing both ancient and recent stratified rocks. They are rarely very remote from a continent, and they always contain some land mammals and amphibia, as well as representatives of the other classes and orders in considerable variety. They may, however, be divided into two well-marked groups—ancient and recent continental islands—the characters of which may be easily defined.

Recent continental islands are always situated on submerged banks connecting them with a continent, and the depth of the intervening sea rarely exceeds 100 fathoms. They resemble the continent in their geological structure, while their animal and vegetable productions are either almost identical with those of the continent, or if otherwise, the difference consists in the presence of closely allied species of the same types, with occasionally a very few peculiar genera. They possess in fact all the characteristics of a portion of the continent, separated from it at a recent geological period.

Ancient continental islands differ greatly from the preceding in many respects. They are not united to the adjacent continent by a shallow bank, but are usually separated from it by a depth of sea of several hundreds to more than a thousand fathoms. In geological structure they agree generally with the more recent islands; like them they possess mammalia and amphibia, usually in considerable abundance, as well as all other classes of animals; but these are highly peculiar, almost all being distinct species, and many forming distinct and peculiar genera or families. They are also well characterised by the fragmentary nature of their fauna, many of the most characteristic continental orders or families being quite unrepresented, while some of their animals are allied, not to such forms as inhabit the adjacent continent, but to others found only in remote parts of the world. This very remarkable set of characters marks off the islands which exhibit them as a distinct class, which often present the greatest anomalies and most difficult problems to the student of distribution.

Oceanic Islands.—The total absence of warm-blooded terrestrial animals in an island otherwise well suited to maintain them, is held to prove that such island is no mere fragment of any existing or submerged continent, but one that has been actually produced in mid-ocean. It is true that if a continental island were to be completely submerged for a single day and then again elevated, its higher terrestrial animals would be all destroyed, and if it were situated at a considerable distance from land it would be reduced to the same zoological condition as an oceanic island. But such a complete submergence and re-elevation appears never to have taken place, for there is no single island on the globe which has the physical and geological features of a continental, combined with the zoological features of an oceanic island. It is true that some of the coral-islands may be formed upon submerged lands of a continental character, but we have no proof of this; and even if it were so, the existing islands are to all intents and purposes oceanic.

We will now pass on to a consideration of some of the more interesting examples of these three classes, beginning with oceanic islands.

All the animals which now inhabit such oceanic islands must either themselves have reached them by crossing the ocean, or be the descendants of ancestors who did so. Let us then see what are, in fact, the animal and vegetable inhabitants of these islands, and how far their presence can be accounted for. We will begin with the Azores, or Western Islands, because they have been thoroughly well explored by naturalists, and in their peculiarities afford us an important clue to some of the most efficient means of distribution among several classes of animals.

title: Island Life/XI


text: Pacchiarotto/Cenciaja

title: Pacchiarotto/Cenciaja


text:
Page:The new British province of South Australia.djvu/42

Rh wish to estimate fully the advantages of his discoveries.

Such extracts only have been made here, as are necessary to throw light on the tract of country immediately under investigation.

Account of Soil, &c.—From the 132° of East long, to Coffin's Bay situated in 135° 15', nothing very important is known of the character of the shore or land. Some large inlets called Fowler's Bay, Denial Bay, Smoky Bay, and Streaky Bay, and a large lagoon seen by Captain Flinders from the mast-head near Point Weyland, are the only indications of valuable roadsteads or rivers.

These inlets have never been thoroughly examined, though Captain Flinders remarks he found in one part "much refuse from the shore, as well as sea-weed floating about, by which some hope of finding a river was entertained;" and subsequently, "besides quantities of grass and branches of trees or bushes floating in the water, there was a number of long gauze-winged insects topping about the surface, such as frequent freshwater lakes and swamps." He also saw smokes rising in various places. In proof of the insufficiency of his survey, Captain Flinders says, "my examination was tolerably minute to be done wholly in a ship, but much still remained which boats would best accomplish, to make the surveys complete, especially in the bays of the main land."

The only account given of Coffin's Bay is by Captain Flinders; we cannot therefore do better than quote the entire passage from his work, vol. i. p. 127.

"Coffin's Bay extends four or five leagues to the south-eastward from Point Sir Isaac; but I do not think that any stream more considerable than perhaps a small rill from the back land falls into it, since sandy cliffs and beach were seen nearly all round. On the east side of the entrance, the shore rises quickly from the beach to hills of considerable height, well covered with wood. The highest of these hills I called Mount


text:
User talk:79.41.154.128

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Rh artillery fire and holding defences that were breastworks rather than trenches; and at nightfall the Russians drew back into the forests, having suffered enormous casualties without reaching the enemy's trenches at any point. Attacks on the village of Vileity, held by the right of the German 3rd Cav. Div., were equally futile. On the Naroch–Viszniev front also the assaults were fierce but disunited, and here too the artillery of the German 75th Res. Div. and 9th Cav. Div. could focus its efforts on each assailant in turn, even that of the III. Res. Corps S. of Lake Viszniev coöperating at times. In sum, the Russians, on the first day, suffered useless and terrible losses in regimental assaults delivered against steady infantry, uncut wire and skilfully handled artillery.

For the following night and day, the Russians changed their tactics. The artillery devoted itself to the demolition of trenches, to wire-cutting, and to the harassing of the billets in the villages behind the defenders' lines, with frequent small infantry attacks intended to force the defence to man its trenches and to march its reserves hither and thither. In this policy they were to some extent successful; the first of the German reinforcing divisions to arrive, the 107th, was put in piecemeal to stiffen the Vileity–Moscheiki front. Outside the battle-field, Russian threats at Vidzy, at Jakobstadt and elsewhere grew more serious. Then, in the night of March 19–20 massed attacks were delivered on the Vileity–Moscheiki front.

The weather conditions both for attackers and defenders had now become terrible. On March 15 a thaw had set in, which, but for Verdun, would probably have caused Ragosa to postpone the whole operation. By March 20 it had reached such a point that the ice on the lakes was covered by 2 ft. of water, while the German trenches in the marshes, no longer pumped out, were waist-deep, and the communications were mere mud. Exhausting as were these conditions for the German soldier, they were paralyzing for the Russian staff. In the forests, which were not seamed with tracks like an Argonne or a Bois le Prêtre, formation for attack and transmission of orders and supply became almost impossible. The night attack on Vileity and on the woods near Moscheiki was utter confusion for both sides. Part of the German defence system was overrun in the first assault, but in the haphazard, frequently hand-to-hand, fighting that followed, superior cohesion and coöperation defeated superior numbers, and the Germans regained the lost trenches, with the aid of parts of the 107th Div. On March 20 the Germans began to receive further reinforcements, the 86th Div. and half of the 85th Div. (170th Bde.). These, however, were held for the protection of the centre and the S. front of the salient, and only the 80th Res. Div. was moved somewhat to the north.

On the night of March 20–1 the night assault was repeated, this time with larger numbers and simultaneously on both the battle fronts. On the N. flank, the assault swept over parts of the defences as before and penetrated deep into the marsh-woods, seeking especially to drive S. and S.W. on to the higher ground behind Postavy. Again resolute counter-attacks stopped its progress, but this time the Russians retained possession of the captured front trenches. On the land-bridge S. of Lake Naroch, a wild assault swept completely over the German 75th Res. Div. holding the “apex,” and it was with difficulty that the defenders' line was reconstituted some kilometres farther back. Only on the Viszniev land-bridge was the assault definitely repulsed. The situation for the Germans became extremely critical. But again it was saved by counter-attack. On March 21 the last forces of the 107th Div., with the exhausted 42nd Div., retook the marsh trenches from the equally exhausted Russians; and on the Naroch land-bridge the putting-in of the whole 80th Div. (brought back from the N.), with parts of the 170th Bde. and 86th Div., not only stabilized the defence but gave it the upper hand. Then it became possible to relieve the exhausted 42nd and 75th Res. Divs. by fresh troops.

The battle continued for a week longer, on the same lines as in the critical days, but with decreasing intensity on the part of the Russians. Presently the lost “apex” was recovered by the Germans, and nearly a month later a local attack still further improved the position on the Naroch land-bridge. But by that time the whole front had become quiet. The last severe battle-day was March 26; after that date the Russian relief-offensive expired without having caused one German soldier to be brought over from France. The German Eastern Headquarters had passed through a period of extreme anxiety, and it is arguable that on March 17–8 they were taken by surprise. But, if so, their recovery was instant, and they managed to meet the calls of the defence out of their local reserves. For the Russians, the first offensive of the new armies was a disaster of far-reaching importance. Prepared, up to the moment of launching, with great adroitness, it had been “choked in blood and marsh” with an enormous cost in mass-casualties and mass-disillusionment.

 NATAL (see 19.252).—At the 1911 census the pop. of Natal, S. Africa, was 1,194,043, of whom 98,114 were whites, 953,398 Bantu, 133,439 Asiatics and 9,092 of mixed or other coloured races. Compared with 1904 the white pop.—which between 1891 and 1904 had nearly doubled—was practically stationary; there was an actual increase of 1,005. In 1918 a census of the whites showed that they numbered 121,931, evidence that the check in their increase had been temporary only. Natal, though the smallest, is the most densely populated province of the Union, with 37.40 persons to the sq.m. in 1918. The white and Asiatic pop. is mainly concentrated in Natal proper; of the Bantu 214,969 lived in Zululand at the 1911 census. Of the total coloured pop. in 1911 only 13.84% were returned as Christians (compared with 44.20% in the Cape). The chief towns were Durban (89,998) and Pietermaritzburg (30,555; in 1919 35,322). Ladysmith ranked next with 5,594 inhabitants. Pietermaritzburg, the capital, has handsome public buildings, including those of the provincial council and Natal University College.

The change from the status of a self-governing colony to a province of the Union affected Natal politically more closely than any other province since in it alone were the great majority of the white inhabitants of British descent. In the first Parliament of the Union the Natal members took an independent position, and the firm attachment of Natalians to the British connexion continued an unchanging factor in the S. African situation. Provincial administration was, however, carried on upon non-party lines (for the provincial system of administration see ). The first administrator was Mr. C. J. Smythe, who had previously held office as Colonial Secretary and as Prime Minister of Natal. Mr. Smythe, who was reappointed for a second term in 1905, died in 1918 and Mr. G. T. Plowman succeeded to the post. The revenue raised in the province, derived chiefly from transfer duties and licences, increased from £118,000 in 1912–3 to £172,000 in 1917–8, the subsidies from the Union Government varying from £361,000 to £375,000. Over half the total expenditure was on education, the sums spent for that object rising from £169,000 in 1912–3 to £285,000 in 1917–8.

Natal was deeply interested in the question of Indians in S. Africa. Of the 152,309 Asiatics in the Union in 1911, no fewer than 149,791 were British Indians and of these 133,048 lived in Natal, where they had rendered possible the development of the sugar, tea and wattle industries, as well as providing labour for the coal-mines, railways and other public works. Besides labourers, there were many Indians engaged in professions and commerce. White S. Africans in general opposed the further increase of Asiatics in the Union; while, in 1911, the Indian Government, long dissatisfied with the attitude of Natal to Indians, prohibited the recruitment of indentured coolies. The Indians both in Natal and the Transvaal complained of many grievances, among them of the poll tax imposed in Natal on all non-indentured Indians. Their cause was championed by Mr. M. K. Ghandi, then resident in S. Africa. Arising out of the agitation, riots and disturbances occurred in Natal in 1913. Some 2,700 Indians started to march to Johannesburg. About 500 were stopped on the border; the rest entered the Transvaal, but were eventually induced to return. In 1914 the poll tax on Indians in Natal was abandoned while the Union passed legislation designed to prevent, with some few exceptions, the entry of further adult male Asiatics into S. Africa and to restrict Asiatics to the provinces in which they were resident. The so-called Smuts-Ghandi agreement of the same year was designed to guard the vested interests of Indians already in the Union (see, further, ).

A notable element in the progress of Natal has been the development of coal-mining. The output, which in 1910 first exceeded 2,500,000 tons, rose to over 3,000,000 in 1916, but fell to 2,600,000

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 tons in 1918, the decline being attributed to the effect of the influenza epidemic of that year. In 1919 the output was 2,800,000 tons. Natal coal is of excellent quality, and commands high prices—double that of Transvaal coal. In 1918 the output was valued at £1,358,000.

 NATHAN, ERNESTO (1845–1921), Italian politician, was born in London in 1845, and died at Rome on April 9 1921. He was the son of an Englishman, Joseph Nathan, and of an Italian mother, Sara Rosselli, both Jews. His parents had befriended Italian political exiles in England, and on the death of Joseph Nathan in 1858 the widow and son settled at Pisa, where the latter attended the university. Soon afterwards they had to repair to Switzerland on account of Sara Nathan's republican sentiments; it was then that Ernesto Nathan became acquainted with Mazzini, whose views became thenceforth his chief inspiration and cult, and he devoted himself as a journalist, teacher and social reformer to their diffusion. A violent anti-clerical, he soon joined the free-masons and was elected “Grand Orient” for Italy in 1899, but resigned in 1905 owing to internal disagreements. He became an Italian citizen, and although he had been a republican in his early years, he gradually accepted the monarchy as the best régime for Italy, and ended by being received at Court. He showed great activity in organizing the “Unione dei Partiti popolari” in 1900, a blocco of the various radical and anti-clerical parties in Rome, and when at the municipal elections of 1900 the clerical administration fell, Nathan was chosen as mayor. That an English Jew and a militant anti-clerical and freemason should become mayor of Rome seemed indeed incongruous, but he was selected for his sterling honesty and business ability. Unfortunately he lost no occasion to offend Catholic sentiment and frequently made himself ridiculous, becoming a butt for the comic papers and revues. His plans for modernizing Rome did much to spoil its beauty for no useful purpose. Reelected in 1910, he fell when the blocco broke up in 1913. On the outbreak of the World War, in spite of his 70 years, he volunteered for the army and actually served as a lieutenant of infantry for a time. In 1917 he was reelected “Grand Orient” but resigned a year later. He was editor of the National edition of Mazzini's works.  NATIONAL DEBT.—The World War, 1914–1918, brought about a complete transformation in the size and composition of the world's national debts. Those of the belligerent countries were swollen to an enormous extent. The liabilities of the European nations were inflated to a degree which in the pre-war period would have been regarded as symptomatic of financial madness and world-wide collapse of credit. The British Prime Minister (Mr. Lloyd George), at the close of the war, estimated its cost at about 40,000 millions sterling, a figure which was accepted by a number of statisticians in Europe and America. Table I. is compiled from Paper No. IV on Public Finance, issued by the League of Nations for the International Financial Conference held at Brussels in the autumn of 1920. For purposes of comparison pre-war figures are also given, when they are available.

The increases shown in Table I., however, cannot be regarded as mathematically correct. Like is not compared with like; the unit of value has been changed in many cases, inconvertible paper currency having been made the legal measure of value in place of a definite weight of gold in all the belligerent countries except the United States and Japan. In order to make a proper comparison it is necessary to make a correction for the depreciation expressed in gold, in the value of money in the various countries. There is, however, no mathematical formula for making this correction, but the existence of a 20% discount in the gold value of the pound sterling (in May 1921) shows that the margin of error is a very material one. From the standpoint of national finance the importance of these national debt figures depends upon their ratio to national revenue. Ratios are shown in Table II., which expresses the debt as so many years' purchase of current revenue.

The countries are arranged in Table II. according to the post-war percentages. As the true burden of debt can only be determined by the relation which it bears to capacity to carry it, it follows that Table II. gives a truer picture of the world's national debt position in 1920 than Table I.

In the case of certain countries such as Austria, Hungary, Poland, and Russia, where the depreciation of the unit of value

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text: 1922 Encyclopædia Britannica/Natal

title: 1922 Encyclopædia Britannica/Natal