the a priori probability of the preservation of at least a few minutely transitional forms in some instances if every species without exception has arisen exclusively by such minute and gradual transitions.

It remains, then, to turn to the other considerations with regard to the relation of species to time: namely (1), as to the total amount of time allowable by other sciences for organic evolution; and (2) the proportion existing, on Darwinian principles, between the time anterior to the earlier fossils, and the time since; as evidenced by the proportion between the amount of evolutionary change during the latter epoch and that which must have occurred anteriorly.

Sir William Thomson has lately' advanced arguments from three distinct lines of inquiry, and agreeing in one approximate result. The three lines of inquiry were-1. The action of the tides upon the earth's rotation. 2. The probable length of time during which the sun has illuminated this planet; and 3. The temperature of the interior of the earth. The result arrived at by these investigations is a conclusion that the existing state of things on the earth, life on the earth, all geological history showing continuity of life, must be limited within some such period of past time as one hundred million years. The first question which suggests itself, supposing Sir W. Thomson's views to be correct, is, Is this period any thing like enough for the evolution of all organic forms by "Natural Selection?" The second is, Is this period any thing like enough for the deposition of the strata which must have been deposited if all organic forms have been evolved by minute steps, according to the Darwinian theory?

In the first place, as to Sir William Thomsom's views, the author of this book cannot presume to advance any opinion; but the fact that they have not been refuted, pleads

9 "Transactions of the Geological Society of Glasgow," vol. iii.

11

strongly in their favor when we consider how much they tell against the theory of Mr. Darwin. The last-named author only remarks that "many of the elements in the calculation are more or less doubtful." 10 and Prof. Huxley does not attempt to refute Sir W. Thomson's arguments, but only to show cause for suspense of judgment, inasmuch as the facts may be capable of other explanations.

Mr. Wallace, on the other hand,12 seems more disposed to accept them, and, after considering Sir William's objections and those of Mr. Croll, puts the probable date of the beginning of the Cambrian deposits 13 at only twenty-four million years ago. On the other hand, he seems to consider that specific change has been more rapid than generally supposed, and exceptionally stable during the last score or so of thousand years.

Now, first, with regard to the time required for the evolution of all organic forms by merely accidental, minute, and fortuitous variations, the useful ones of which have been preserved.

14

Mr. Murphy 1 is distinctly of opinion that there has not been time enough. He says: "I am inclined to think that geological time is too short for the evolution of the higher forms of life out of the lower by that accumulation of imperceptibly slow variations, to which alone Darwin ascribes the whole process. "3

"Darwin justly mentions the greyhound as being equal to any natural species in the perfect coördination of its parts, all adapted for extreme fleetness and for running down weak prey.'" "Yet it is an artificial species (and not physiologically a species at all), formed by long-con

10 "Origin of Species," 5th edit., p. 354.

11 See his address to the Geological Society, on February 19, 1869. 12 See Nature, vol. i., p. 399, February 17, 1870.

13 Ibid., vol. i., p. 454.

14 "Habit and Intelligence," vol. i., p. 344.

tinued selection under domestication; and there is no reason to suppose that any of the variations which have been selected to form it have been other than gradual and almost imperceptible. Suppose that it has taken five hundred years to form the greyhound out of his wolf-like ancestor. This is a mere guess, but it gives the order of the magnitude." Now, if so, "how long would it take to obtain an elephant from a protozoon, or even from a tadpolelike fish? Ought it not to take much more than a million times as long?

15 19

16

Mr. Darwin 1 would compare with the natural origin of a species “unconscious selection, that is, the preservation of the most useful or beautiful animals, with no intention of modifying the breed." He adds: "But by this process of unconscious selection, various breeds have been sensibly changed in the course of two or three centuries."

"Sensibly changed!" but not formed into “ new species." Mr. Darwin, of course, could not mean that species generally change so rapidly, which would be strangely at variance with the abundant evidence we have of the stability of animal forms as represented on Egyptian monuments and as shown by recent deposits. Indeed, he goes on to say: "Species, however, probably change much more slowly, and within the same country only a few change at the same time. This slowness follows from all the inhabitants of the same country being already so well adapted to each other, that places in the polity of Nature do not occur until after long intervals, when changes of some kind in the physical conditions, or through immigration, have occurred, and individual differences and variations of the right nature, by which some of the inhabitants might be better fitted to their new places under altered circumstances, might not at once occur." This is true, and not only will 15 “Habit and Intelligence," vol. i., p. 345, 16" Origin of Species," 5th edit., p. 353.

these changes occur at distant intervals, but it must be borne in mind that in tracing back an animal to a remote ancestry, we pass through modifications of such rapidly-increasing number and importance that a geometrical progression can alone indicate the increase of periods which such profound alterations would require for their evolution through "Natural Selection" only.

Thus let us take for an example the proboscis monkey of Borneo (Semnopithecus nasalis). According to Mr. Darwin's own opinion, this form might have been "sensibly changed" in the course of two or three centuries. According to this, to evolve 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 Cebidae-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 difference 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, for 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 vertebrate 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 jelly-fish), 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, in this matter geology must in some respects 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 that 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 Verte