Tertiary formations.  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.  A 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 time from the geologist.  The geologist, considering the rate at which deposits are formed and the rate at which denudation goes on upon the surface of the earth, arrives at more or less justifiable conclusions as to the time which is required for the deposit of a certain thickness 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 a 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—­tells me that my geological authority is quite wrong; and that he has weighty evidence to show that life could not possibly have existed upon the surface of the earth 500,000,000 years ago, because the earth would have then 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 among yourselves I will adopt your conclusion.”  We take our time from the geologists and physicists; 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 too fast or too slow.  What we desire to know is, is it a fact that evolution took place?  As to the amount of time which evolution may have occupied, we are in the hands of the physicist and the astronomer, whose business it is to deal with those questions.

* * * * *

I have now, ladies and gentlemen, arrived at the conclusion of the task which I set before myself when I undertook to deliver these lectures.  My purpose has been, not to enable those among you who have paid no 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 I have desired to put before you the principles upon which all hypotheses respecting the history of Nature must be judged; and furthermore, to make apparent the nature of the evidence and the amount 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 shrunk from taking you through long discussions, that I fear may have sometimes tried your patience; and I have inflicted upon you details which were indispensable, but which may well have been wearisome.  But I shall rejoice—­I shall consider that I have done you the greatest service, which it was in my power to do—­if I have thus convinced you that the great question which we have been discussing is not one to be dealt with by rhetorical flourishes, or by loose and superficial talk; but that it requires the keen attention of the trained intellect and the patience of the accurate observer.