There remains the third hypothesis, that of which I have spoken as the hypothesis of evolution; and I purpose that, in lectures to come, we should discuss it as carefully as we have considered the other two hypotheses.  I need not say that it is quite hopeless to look for testimonial evidence of evolution.  The very nature of the case precludes the possibility of such evidence, for the human race can no more be expected to testify to its own origin, than a child can be tendered as a witness of its own birth.  Our sole inquiry is, what foundation circumstantial evidence lends to the hypothesis, or whether it lends none, or whether it controverts the hypothesis.  I shall deal with the matter entirely as a question of history.  I shall not indulge in the discussion of any speculative probabilities.  I shall not attempt to show that Nature is unintelligible unless we adopt some such hypothesis.  For anything I know about the matter, it may be the way of Nature to be unintelligible; she is often puzzling, and I have no reason to suppose that she is bound to fit herself to our notions.

I shall place before you three kinds of evidence entirely based upon what is known of the forms of animal life which are contained in the series of stratified rocks.  I shall endeavour to show you that there is one kind of evidence which is neutral, which neither helps evolution nor is inconsistent with it.  I shall then bring forward a second kind of evidence which indicates a strong probability in favour of evolution, but does not prove it; and, lastly, I shall adduce a third kind of evidence which, being as complete as any evidence which we can hope to obtain upon such a subject, and being wholly and strikingly in favour of evolution, may fairly be called demonstrative evidence of its occurrence.

II

THE HYPOTHESIS OF EVOLUTION.  THE NEUTRAL AND THE FAVOURABLE EVIDENCE

In the preceding lecture I pointed out that there are three hypotheses which may be entertained, and which have been entertained, respecting the past history of life upon the globe.  According to the first of these hypotheses, living beings, such as now exist, have existed from all eternity upon this earth.  We tested that hypothesis by the circumstantial evidence, as I called it, which is furnished by the fossil remains contained in the earth’s crust, and we found that it was obviously untenable.  I then proceeded to consider the second hypothesis, which I termed the Miltonic hypothesis, not because it is of any particular consequence whether John Milton seriously entertained it or not, but because it is stated in a clear and unmistakable manner in his great poem.  I pointed out to you that the evidence at our command as completely and fully negatives that hypothesis as it did the preceding one.  And I confess that I had too much respect for your intelligence to think it necessary to add that the negation was equally clear