The discussion of the distribution of animals and plants, when carried on in the light of geology, raises many interesting questions as to time, which we have already glanced at, but which deserve a little more attention. As to the vast duration of geological time all geologists are agreed. It is, however, now well understood that science sets certain limits to the time at our disposal. Edward Forbes humorously defined a geologist to be "an amiable enthusiast who is content if allowed to appropriate as much as he pleases of that which other men value least, namely, past time "; but now even the geologist is obliged to be content with a limited quantity of this commodity.
The well-known estimate of Lord Kelvin gave one hundred millions of years as the probable time necessary for the change of the earth from the condition of a molten mass to that which we now see. On this estimate we might fairly have assumed fifty millions of years as covering the time from the Laurentian age to the modern period. The great physicist has, however, after allowing us thus much credit in the bank of time, "suddenly put up the shutters and declared a dividend of less than four shillings in the pound."[187] In other words, he has reduced the time at our disposal to twenty millions of years. Other physicists, reasoning on the constitution of the sun, agree with this latter estimate, and affirm that "twenty millions of years ago the earth was enveloped in the fiery atmosphere of the sun."[188] Geology itself has attempted an independent calculation based on the wearing down of our continents, which appears to proceed at the rate of about a foot in four or five thousand years, and on the time required to deposit the sediments of the several geological formations, estimated at about 70,000 feet in thickness. These calculations would give us, say, eighty-six millions of years since the earth began to have a solid crust, which would, like Lord Kelvin's earlier estimate, give us nearly fifty millions of years for the geological time since the introduction of life. The details of the several estimates made it would be tedious and unprofitable to enter into, but I may state as my own conclusion, that the modern rates of denudation and deposit must be taken as far below the average, and that perhaps the estimate stated by Wallace on data supplied by Houghton, namely, twenty-eight millions, may be not far from the truth, though perhaps admitting of considerable abatement.
[187] Bonney, Address before British Association, 1888.
[188] Newcomb, Helmholtz, Tait, etc.
This reduced estimate of geological time would still give scope enough for the distribution of animals and plants, but it will scarcely give that required by certain prevalent theories of evolution. When Darwin says, "If the theory (of natural selection) 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 to the present day," he makes a demand which geology cannot supply; for independently of our ignorance of any formations or fossils, except those included in the Archæan, to represent this vast succession of life, the time required would push us back into a molten state of the planet. This difficulty is akin to that which meets us with reference to the introduction of many and highly specialized mammals in the Eocene, or of the forests of modern type in the Cretaceous. To account for the origin of these by slow and gradual evolution requires us to push these forms of life so far back into formations which afford no trace of them, but, on the contrary, contain other creatures that appear to be exclusive of them, that our faith in the theory fails. The only theory of evolution which seems to meet this difficulty is that advanced by Mivart, Leconte, and Saporta, of "critical periods," or periods of rapid introduction of new species alternating with others of comparative inaction. This would much better accord with the apparently rapid introduction of many new forms of life over wide regions at the same period. It would also approach somewhat near, in its manner of stating the problem to be solved, to the theory of "creation by law" as held by the Duke of Argyll, or to what may be regarded as "mediate creation," proceeding in a regular and definite manner, but under laws and forces as yet very imperfectly known, throughout geological time.
It seems singular, in view of the facts of palæontology, that evolutionists of the Darwinian school are so wedded to the idea of one introduction only of each form of life, and its subsequent division by variation into different species, as it progressively spreads itself over the globe, or is subjected to different external conditions. It is evident that a little further and very natural extension of their hypothesis would enable them to get rid of many difficulties of time and space. For example, certain Millipedes and Batrachians are first known in the coal formation, and this not in one locality only, but in different and widely separated regions. If they took beginning in one place, and spread themselves gradually over the world, this must have required a vast lapse of time—more than we can suppose probable. But if, in the coal-formation age, a worm could anywhere change into a Millipede, or a fish into a Batrachian, why might this not have occurred in many places at once? Again, if the oldest known land snails occur in the coal formation, and we find no more specimens till a much later period, why is it necessary to suppose that these creatures existed in the intervening time, and that the later species are the descendants of the earlier? Might not the process have been repeated again and again, so as to give animals of this kind to widely separated areas and successive periods without the slow and precarious methods of continuous evolution and migration? This apparent inconsistency strikes one constantly in the study of discussions of the theory of derivation in connection with geographical and geological distribution. We constantly find the believers in derivation laboriously devising expedients for the migration of animals and plants to the most unlikely places, when it would seem that they might just as well have originated in those places by direct evolution from lower forms. Those who believe in a separate centre of creation for each species must of course invoke all geological and geographical possibilities for the dispersion of animals and plants; but surely the evolutionist, if he has faith in his theory, might take a more easy and obvious method, especially when in any case he is under the necessity of demanding a great lapse of time. That he does not adopt this method perhaps implies a latent suspicion that he must not repeat his miracle too often. He also perceives that if repeated and unlimited evolution of similar forms had actually occurred, there could have remained little specific distinctness, and the present rarity of connecting links would not have occurred. Further, a new difficulty would have sprung up in the geographical and geological relations of species and genera, which would then have assumed too much of the aspect of a preconceived plan. It is only fair to a well-known and somewhat extreme European evolutionist, Karl Vogt, to state that he launches boldly into the ocean of multiple evolution, not fearing to hold that identical species of mollusks have been separately evolved in separate Swiss lakes, and that the horse has been separately evolved in America and in Europe, in the former along a line beginning with Eohippus, and in the latter along an entirely separate line, commencing with Palæotherium. The serious complications resulting from such admissions are evident, but Vogt deserves credit for faith and consistency beyond those of his teachers.
With reference to the actual distribution of species, the question of time becomes most important when applied to the Glacial period, since it is obvious that much of the present distribution must have been caused, or greatly modified, by that event. The astronomical theory would place the close of the Glacial age as far back as 70,000 or 80,000 years ago. But we have already seen in the chapter on that period that geological facts bring its close to only from 10,000 to 7,000 years before our time. If we adopt the shorter estimates afforded by these facts, it will follow that the submergences and emergences of land in the Glacial ages were more rapid than has hitherto been supposed, and that this would react on our estimate of time by giving facilities for more rapid denudation and deposition. Such results would greatly shorten the duration assignable to the human period. They would render it less remarkable that no new species of animals seem to have been introduced since the Glacial age, that many insular faunas belong to far earlier times, and that no changes even leading to the production of well-marked varieties have occurred in the post-glacial or modern age.
In conclusion, does all this array of fact and reasoning bring us any nearer to the comprehension of that "mystery of mysteries," the origin and succession of life? It certainly does not enable us to point to any species, and to say precisely here, at this time and thus it orginated. If we adopt the theory of evolution, the facts seem to restrict us to that form of it which admits paroxysmal or intermittent introduction of species, depending on the concurrence of conditions favourable to the action of the power, whatever it may be, which produces new organisms. Nor is there anything in the facts of distribution to invalidate the belief in creation, according to definite laws, if that really differs in its nature from certain forms of the hypothesis of evolution. We have also learned that, time being given, animals and plants manifest wonderful powers of migration, that they can vary within considerable limits without ceasing to be practically the same species, and that under certain conditions they can endure far longer in some places than in others. We also see evidence that it is not on limited islands, but on the continents, that land animals and plants have originated, and that swarms of new and vigorous species have issued from the more northern regions in successive periods of favourable Arctic climate. The last of these new swarms or "centres of creation," that with which man himself is more closely connected, belongs to the Palearctic region. We have already seen that in every geological period, when the submerged continental plateaus were pervaded by the warm equatorial waters, multitudes of new marine species appear. In times when, on the contrary, the colder Arctic currents poured over these submerged surfaces, carrying mud and stones, great extinction took place, but certain northern forms of life swarmed abundantly, and when elevation took place, marine species became extinct or were forced to migrate. Everywhere and at all times multiplication of species was promoted by facilities for expansion. The great limestones of our continents, full of corals and shells of new species, belong to times when the ocean spread itself over the continental plateaus, affording wide, untenanted areas of warm and shallow water. The introduction of new faunas and floras on the land belongs to times when vast supplies of food for plants and animals and favourable conditions of existence were afforded by the emergence of new lands possessing fertile soils and abundantly supplied with light, heat, and moisture. Thus geological and geographical facts concur with ordinary observation and experience in reference to varietal forms, in testifying that it is not mere struggle for existence, but facilities for easy existence and rapid extension, that afford the conditions necessary for new and advanced forms of life. These considerations do not, of course, reach to the first cause of the introduction of species, nor even to the precise mode in which this may have acted in any particular case: but perhaps we cannot fully attain to this by any process of inductive inquiry. The study of geographical distribution,' therefore, does not enable us to solve the question of the origin of specific types, but, on the contrary, points to marvellous capacities for migration and a wonderful tenacity of life in species. In these respects, however, it is a study full of interest, and in nothing more so than in the evidence which it affords of the practically infinite provisions made for the peopling of every spot of land or sea with creatures fitted to flourish and enjoy life therein, and to carry on the great and progressive plan of the Creator.
References:—Continental and Island Life, Princeton Review, July, 1881. Address to American Association, 1883. Papers and Addresses to Natural History Society, Canadian Naturalist, Montreal. "The Story of the Earth and Man," 1st ed., 1873, 9th ed., London, 1887.