There is no doubt as to the meaning of this statement, nor as to what a man of Milton’s genius expected would have been actually visible to an eye-witness of this mode of origination of living things.

The third hypothesis, or the hypothesis of evolution, supposes that, at any comparatively late period of past time, our imaginary spectator would meet with a state of things very similar to that which now obtains; but that the likeness of the past to the present would gradually become less and less, in proportion to the remoteness of his period of observation from the present day; that the existing distribution of mountains and plains, of rivers and seas, would show itself to be the product of a slow process of natural change operating upon more and more widely different antecedent conditions of the mineral framework of the earth; until, at length, in place of that framework, he would behold only a vast nebulous mass, representing the constituents of the sun and of the planetary bodies.  Preceding the forms of life which now exist, our observer would see animals and plants not identical with them, but like them; increasing their differences with their antiquity and, at the same time, becoming simpler and simpler; until, finally, the world of life would present nothing but that undifferentiated protoplasmic matter which, so far as our present knowledge goes, is the common foundation of all vital activity.

The hypothesis of evolution supposes that in all this vast progression there would be no breach of continuity, no point at which we could say “This a natural process,” and “This is not a natural process;” but that the whole might be compared to that wonderful process of development which may be seen going on every day under our eyes, in virtue of which there arises, out of the semi-fluid, comparatively homogeneous substance which we call an egg, the complicated organization of one of the higher animals.  That, in a few words, is what is meant by the hypothesis of evolution.

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I have already suggested that in dealing with these three hypotheses, in endeavouring to form a judgment as to which of them is the more worthy of belief, or whether none is worthy of belief—­in which case our condition of mind should be that suspension of judgment which is so difficult to all but trained intellects—­we should be indifferent to all a priori considerations.  The question is a question of historical fact.  The universe has come into existence somehow or other, and the problem is, whether it came into existence in one fashion, or whether it came into existence in another; and, as an essential preliminary to further discussion, permit me to say two or three words as to the nature and the kinds of historical evidence.

The evidence as to the occurrence of any event in past time may be ranged under two heads which, for convenience’ sake, I will speak of as testimonial evidence and as circumstantial evidence.  By testimonial evidence I mean human testimony; and by circumstantial evidence I mean evidence which is not human testimony.  Let me illustrate by a familiar example what I understand by these two kinds of evidence, and what is to be said respecting their value.