opinion, this form might have been "sensibly changed" in the course of two or three centuries. According to this, for evolving it as a true and perfect species one thousand years would be a very moderate period. Let ten thousand years be taken to represent approximately the period of substantially constant conditions during which no considerable change would be brought about. Now, if one thousand years may represent the period required for the evolution of the species S. nasalis, and of the other species of the genus Semnopithecus; ten times that period should, I think, be allowed for the differentiation of that genus, the African Cercopithecus and the other genera of the family Simiida-the differences between the genera being certainly more than tenfold greater than those between the species of the same genus. Again we may perhaps interpose a period of ten thousand years' comparative repose.

For the differentiation of the families Simiidæ and Cebidæ so very much more distinct and different than any two genera of either family-a period ten times greater should, I believe, be allowed than that required for the evolution of the subordinate groups. A similarly increasing ratio should be granted for the successive developments of the differences between the Lemuroid and the higher forms of primates; for those between the original primate and other root-forms of placental mammals; for those between primary placental and implacental mammals, and perhaps also for the divergence of the most ancient stock of these and of the monotremes; since in all these cases modifications of structure appear to increase in complexity in at least that ratio. Finally, a vast period must be granted for the development of the lowest mammalian type from the primitive stock of the whole verte

brate sub-kingdom. Supposing this primitive stock to have arisen directly from a very lowly organized animal indeed (such as a nematoid worm, or an ascidian, or a jellyfish), yet it is not easy to believe that less than two thousand million years would be required for the totality of animal development by no other means than minute, fortuitous, occasional, and intermitting variations in all conceivable directions. If this be even an approximation to the truth, then there seem to be strong reasons for believing that geological time is not sufficient for such a process.

The second question is, whether there has been time enough for the deposition of the strata which must have been deposited, if all organic forms have been evolved. according to the Darwinian theory?

Now this may at first seem a question for geologists only, but, in fact, geology in this matter must rather take its time from zoology than the reverse; for if Mr. Darwin's theory be true, past time down to the deposition of the Upper Silurian strata can have been but a very small fraction of the whole time during which strata have been deposited. For when those Upper Silurian strata were formed, organic evolution had already run a great part of its course, perhaps the longest, slowest, and most difficult part of that course.

At that ancient epoch not only were the vertebrate, molluscous, and arthropod types distinctly and clearly differentiated, but highly developed forms had been produced in each of these sub-kingdoms. Thus in the Vertebrata there were fishes not belonging to the lowest but to the very highest groups which are known to have ever been developed, namely, the Elasmobranchs (the

highly organized sharks and rays) and the Ganoids, a group now poorly represented, but for which the sturgeon may stand as a type, and which in many important respects more nearly resemble higher Vertebrata than do the ordinary or osseous fishes. Fishes in which the ventral fins are placed in front of the pectoral ones (i.e. jugular fishes) have been generally considered to be comparatively modern forms. But Professor Huxley has obligingly informed the author that he has discovered a jugular fish in the Permian deposits.

Amongst the molluscous animals we have members of the very highest known class, namely, the Cephalopods, or cuttle-fish class; and amongst articulated animals we find Trilobites and Eurypterida, which do not belong to any incipient worm-like group, but are distinctly differentiated Crustacea of no low form.1

We have in all these animal types nervous systems differentiated on distinctly different patterns; fully formed organs of circulation, digestion, excretion, and generation; complexly constructed eyes and other sense organs. In fact we have all the most elaborate and complete animal structures built up, and not only once; for in the fishes and mollusca we have (as described in the third chapter of this work) the coincidence of the independently developed organs of sense attaining a nearly similar complexity in two quite distinct forms. If, then, so small an advance in organization has been made in fishes, molluscs,

1 Dr. Scudder "has lately found a fossil insect in the Devonian formation of New Brunswick, which is furnished with the well-known tympanum or stridulating apparatus of the male Locustida." (Trans. Ent. Soc., Third Series, vol. ii., quoted in Darwin's "Descent of Man," vol. i. p. 360.)

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 admits1 "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.

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