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Show 106 DARWINIAN A. theory will very well account for the origination by divergence of nearly-related species, whether within the present period or in remoter geological times ; a very natural view for him to take, since he appears to have reached and published, several years ago, the pregnant conclusion that there most probably was some material connection between the closely-related species of two successive faunas, and that the numerous close species, whose limits are so difficult to determine, were not all created distinct and independent. But while thus accepting, or ready to accept, the basis of Darwin's theory, and all its legitimate direct inferences, he rejects the ultimate conclusions, brings some·weighty arguments to bear against them, and is evidently convinced that he can draw a clear line between the sound inferences~ which he favors, and the unsound or unwarranted theoretical deductions, which he rejects. We hope he can. This raises the question, Why does Darwin press his theory to these extreme conclusions? Why do aU hypotheses of derivation co!l.verge so inevitably to one ultimate point? Having already considered some of the reasons which suggest or support the t~eory at its outset-which may carry it as far as such sound and experienced naturalists as .Pictet allow that it may be true-perhaps as far as Darwin himself unfolds it in the introductory proposition cited at the beginning of this article-we may now inquire after the de l'Homme Fossile," in the same (March) number of the Bibliotheque Universelle. (See, also, the same author's "Note sur la Periode Quaternaire ou Diluvienne, consider~e dans ses Rapports avec l'Epoque Actuelle," in the number for August, 1860, of the same periodical.) NATURAL SELECTION, ETO. 107 motives which impel the theorist so much farther. Here proofs, in the proper sense of the word, are not to be had. We are beyond the region of demonstration and have only probabilities to consider. What are 'these probabilities? What work will thi~ ~ypothesis do to establish a claim to be adopted m Its completeness? Why should a the~ry ;vhic~ may plausibly enough account for the d1Jversificatwn of the species of each special type or genus be expanded into a general system for the origination ?r successive diversification of all species, and all spemal types or forms from four or five remote primordial forms, or perha~s from one? We accept the theory of gravitation because it explains all the facts we know, and bears all the tests that we can put it to. We incline to accept the nebular hypothesis, f01: s~m~lar reas_:>ns ; not because it is proved-thus far It IS mcapable of p 1·oqf-but because it is a natural theoretical deduction from accepted physical laws, is thoroughly congruous with the facts, and because its assumption serves to connect and harmonize these into one probable and consistent whole. Can the derivative hypothesis be maintained and carried out into a system on similar grounds? If so, however unproved, it would appear to be a tenable hypothesis, which is all that its author ought now to claim. Such hypotheses as, from t~e conditions of the case, can neither be proved nor disproved by direct evidence or experi~ent, are to be tested only indirectly, and therefore Imperfectly, by trying their power to harmoni~e the known facts, and to account for what is otherwise unaccountable. So the question comes to this : What will an hypothesis |
