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and arthropods since the Upper Silurian deposits, it will probably be within the mark to consider that the period before those deposits (during which all these organs would, on the Darwinian theory, have slowly built up their different perfections and complexities) occupied time at least a hundredfold greater.
Now it will be a moderate computation to allow 25,000,000 years for the deposit of the strata down to and including the Upper Silurian. If, then, the evolutionary work done during this deposition only represents a hundredth part of the sum total, we shall require 2,500,000,000 (two thousand five hundred million) years for the complete development of the whole animal kingdom to its present state. Even one quarter of this, however, would far exceed the time which physics and
astronomy seem able to allow for the completion of the process.
Finally, a difficulty exists as to the reason of the absence of rich fossiliferous deposits in the oldest strata-if life was then as abundant and varied as, on the Darwinian theory, it must have been, Mr. Darwin himself admits 1 " the case at present must remain inexplicable; and may be truly urged as a valid argument against the views' entertained in his book.
Thus, then, we find a remarkable (and on Darwinian principles an inexplicable) absence of minutely graduated transitional forms. All the most marked groups-bats, pterodactyles, chelonians, ichthyosauria, anoura, &c.— appear at once upon the scene. Even the horse, the animal whose pedigree has been probably best preserved, affords no conclusive evidence of specific origin by insignificant fortuitous variations; while some forms, as the labyrinthodonts and trilobites, which seemed to exhibit gradual change, are shown by further investigation to do nothing of the sort. As regards the time required for evolution (whether estimated by the probably minimum period required for organic change, or for the deposition of strata which accompanied that change), reasons have been suggested why it is likely that the past history of the earth does not supply us with enough. First, because of the prodigious increase in the importance and number of differences and modifications which we meet with as we traverse successively greater and more primary zoological groups; and, secondly, because of the vast series of strata necessarily deposited if the period since the Lower Silurian marks but a small fraction of the whole
1" Origin of Species,” 5th edition, p. 381.
period of organic - evolution. Finally, the absence or rarity of fossils in the oldest rocks is a point at present inexplicable, and not to be forgotten or neglected.
Now all these difficulties are avoided if we admit that new forms of animal life of all degrees of complexity appear from time to time with comparative suddenness, being evolved according to laws in part depending on surrounding conditions, in part internal-similar to the way in which crystals (and, perhaps from recent researches, the lowest forms of life) build themselves up according to the internal laws of their component substance, and in harmony and correspondence with all environing influences and conditions.
The geographical distribution of animals presents difficulties.--These not
insurmountable in themselves; harmonize with other difficulties. Fresh-water fishes.-Forms common to Africa and India ; to Africa and South America ; China and Australia ; to North America and China ; to New Zealand and South America; to South America and Tasmania ; to South America and Australia.—Pleurodont lizards.- Insectivorous mammals. -Similarity of European and South American frogs.
- Analogy between European salmon and fishes of New Zealand, &c. An ancient Antarctic continent probable. ---Other modes of accounting for facts of distribution.-Independent origin of closely similar forms. - Conclusion.
The study of the distribution of animals over the earth's surface presents us with many facts having certain not unimportant bearings on the question of specific origin. Amongst these are instances which, at least at first sight, appear to conflict with the Darwinian theory of Natural Selection." It is not, however, here contended that such facts do by any means constitute of themselves obstacles which cannot be got over. Indeed, it would be difficult to imagine any obstacles of the kind which could not be surmounted by an indefinite number of terrestrial modifications of surface
surface - submergences and emergences-junctions and separations of continents in all directions, and combinations of any desired degree of frequency.
All this being supplemented by the intercalation of armies of enemies, multitudes of ancestors of all kinds, and myriads of connecting forms, whose raison d'être may be simply their utility or necessity for the support of the theory of “Natural Selection."
Nevertheless, when brought in merely to supplement and accentuate considerations and arguments derived from other sources, in that case difficulties connected with the geographical distribution of animals are not without significance, and are worthy of mention even though by themselves they constitute but simple problems, the solution or non-solution of which could not alone vitally affect any theory of specific origination.
Many facts as to the present distribution of animal life over the world are very readily explicable by the hypothesis of slight elevations and depressions of larger and smaller parts of its surface, but there are others which it is much more difficult so to explain.
The distribution either of animals possessing the power of flight, or of inhabitants of the ocean, is of course easily to be accounted for; the difficulty, if there is really any, must mainly be with strictly terrestrial animals of moderate or small powers of locomotion and with inhabitants of fresh water. Mr. Darwin himself observes,1 " In regard to fish, I believe that the same species never occur in the fresh waters of distant continents.” Now, the author is enabled, by the labours and through the kindness of Dr. Günther, to show that this belief cannot be maintained; that naturalist having called his attention to the following facts with regard to fish-distribution. These facts show that though only one species which is absolutely
1 “Origin of Species,” 5th edition, 1869, p. 463.