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Show 136 MR. DARWIN'S WOll.K A~ D whit more preposterous than the objections which have been made to the "Origin of Species." Mr. Darwin, then, had a perfect right to limit his inquiry as he pleased, and the only question for us-the inquiry being so limited-is to ascertain whether the n1ethod of his inquiry is sound or unso1:1nd ; w hcther he has obeyed the canons which must guide and govern_ all investigation, or whether he has broken them; and it was because our inquiry this evening is essentia.Uy limited to that question, that I spent a good deal of time in a former lecture (which, perhaps some of you thought might have been better employed) in endeavouring to illustrate the method and nature of scientific inquiry in general. vV e shall now have to put in practice the principles that I then laid down. I stated to you in substance, if not in words, that wherever there are complex masses of phenomena to be inquired into, whether they be phenomena of the affairs of daily life, or whether they belong to the more abstruse and difficult problems laid before the philosopher, our course of proceeding in unravelling that complex chain of phenomena with a view to get at its cause, is always the same; in all cases we must invent a hypothesis; we must place before ourselves some more or less likely supposition respecting that cause; and then, having assumed a hypothesis, having supposed a cause for the phenomena in question, we must endeavour, on the one hand, to demonstrate our hypothesis, or, on the other, to upset and reject it altogether, by testing it in three ways. We must, in the first place, be prepared to prove that the supposed causes of the phenomena exist in nature; that they are what the logicians THE PHENOMENA OF ORGANIC NATURE. 137 call vera causm-true causes ;-in the next place, we should be prepared to show that the assumed causes of the phenomena are competent to produce such phenoInena as those which we wish to explain by them ; and in the last place, we ought to be able to show that no other kno·wn causes are competent to produce these phenomena. If we can succeed in satisfying these three conditions we shall have demonstrated our hypothesis; or rather I ought to say, we shall have proved it as far as certainty is possible for us; for, after all, there is no one of our surest convictions which may not be upset, or at any rate modified by a further accession of knowledge. It was because it satisfied these conditions that we accepted the hypothesis as to the disappearance of the tea-pot and spoons in the case I supposed in a previous lecture; we found that our hypothesis on that subject was tenable and valid, because' the supposed cause existed in nature, because it was· competent to account for the phenomena, and because no other known cause was competent to account for them; and it is upon similar grounds that any hypothesis you choose to name is accepted in science as tenable and valid. What is Mr. Darw~n's hypothesis? As I apprehend it-for I have put it into a shap~ more convenient for common purposes than I could find verbatim in his book-as I apprehend it, I say, it is, that all the phenomena of organic nature, past and present, result from, or are caused by, the,inter-action of those properties of organic matter, which we have called ATAVISM and VARIABILITY, '"ith the CoNDI.TIONs 0}~ ExiSTENCE; or, in other '7ords,-given the existence of |