evolution does require so great a time? The biologist knows nothing whatever of the amount of time which may be required for the process of evolution. It is a matter of fact that those forms which I have described to you occur in the order which I have described in the tertiary formation. But I have not the slightest means of guessing whether it took a million of years, or ten millions, or a hundred millions, or a thousand millions of years, to give rise to that series of changes. As a matter of fact, the biologist has no means of arriving at any conclusion as to the amount of time which may be needed for a certain quantity of organic change. He takes his facts as to time from the geologist. The geologist, taking into consideration the rate at which deposits are formed and the rate at which denudation goes on upon the surface of the earth, arrives at certain more or less justifiable conclusions as to the time which is required for the deposit of a certain amount of rocks, and if he tells me that the tertiary formations required 500,000,000 years for their deposit, I suppose he has good ground for what he says, and I take that as the measure of the duration of the evolution of the horse from the Orohippus up to its present condition. And, if he is right, undoubtedly evolution is a very slow process, and requires a great deal of time. But suppose, now, that an astronomer or a physicist—for instance, my friend Sir William Thomson—comes to me and tells me that my geological friend is quite wrong, and that he has capital evidence to show that life could not possibly have existed upon the surface of the earth 500,000,000 of years ago, because the earth would have been too hot to allow of life, my reply is: "That is not my affair; settle that with the geologist, and when you have come to an agreement between yourselves I will adopt your conclusion." We take our time from the geologist, and it is monstrous that, having taken our time from the physical philosopher's clock, the physical philosopher should turn round upon us, and say we are going too fast. What we desire to prove is, is it a fact that evolution took place? As to the amount of time it took, we are in the hands of the physicist and the astronomer, whose business it is to deal with those questions.
I think, ladies and gentlemen, that I have now arrived at the conclusion of the task which I set before myself when I undertook to deliver these lectures before you. My purpose has been, not to enable those of you who have not paid attention to these subjects before to leave this room in a condition to decide upon the validity or the invalidity of the hypothesis of evolution, but to put before you the principles by which all such hypotheses must be judged; and, furthermore, to make apparent to you the nature of the evidence and the sort of cogency which is to be expected and may be obtained from it. To this end I have not hesitated to regard you as genuine students and persons desirous of knowing the truth. I have not hesitated to take you through arguments, even long chains of arguments, that I
