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[[133]] 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 anything like enough for the evolution of all organic forms by "Natural Selection"? The second is, Is this period anything 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 Thomson's views, the Author of this book cannot presume to advance any opinion; but the fact that they have not been refuted, pleads strongly in
their favour 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,"[[134]] and Professor Huxley[[135]] 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,[[136]] 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[[137]] 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:
Mr. Murphy[[138]] 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."
"Darwin justly mentions the greyhound as being equal to any natural species in the perfect co-ordination 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-continued 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 tadpole-like fish? Ought it not to take much more than a million times as long?"[[139]]
Mr. Darwin[[140]] 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."