[Continued.]
"I can entertain no doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists entertain, and which I formerly entertained,—namely, that each species has been independently created,—is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the main, but not exclusive means of modification."
This is the kernel of the new theory, the Darwinian creed, as recited at the close of the introduction to the remarkable book under consideration. The questions, "What will he do with it?" and "How far will he carry it?" the author answers at the close of the volume: "I cannot doubt that the theory of descent with modification embraces all the members of the same class." Furthermore, "I believe that all animals have descended from at most only four or five progenitors, and plants from an equal or lesser number." Seeing that analogy as strongly suggests a further step in the same direction, while he protests that "analogy may be a deceitful guide," yet he follows its inexorable leading to the inference that "probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."[a]
In the first extract we have the thin end of the wedge driven a little way; in the last, the wedge is driven home.
We have already (in the preceding number) sketched some of the reasons suggestive of such a theory of derivation of species,—reasons which give it plausibility, and even no small probability, as applied to our actual world and to changes occurring since the latest tertiary period. We are well pleased at this moment to find that the conclusions we were arriving at in this respect are sustained by the very high authority and impartial judgment of Pictet, the Swiss palaeontologist. In his review of Darwin's book,—much the fairest and most admirable opposing one that has yet appeared,—he freely accepts that ensemble of natural operations which Darwin impersonates under the now familiar name of Natural Selection, allows that the exposition throughout the first chapters seems "à la fois prudent et fort" and is disposed to accept the whole argument in its foundations, that is, so far as it relates to what is now going on, or has taken place in the present geological period,—which period he carries back through the diluvial epoch to the borders of the tertiary.[c] Pictet accordingly admits that the 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 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.
[Footnote a: P. 484, Engl. ed. In the new American edition, (Vide Supplement, pp. 431, 432,) the principal analogies which suggest the extreme view are referred to, and the remark is appended,—"But this inference is chiefly grounded on analogy, and it is immaterial whether or not it be accepted. The case is different with the members of each great class, as the Vertebrata or Articulata; for here we have in the laws of homology, embryology, etc., some distinct evidence that all have descended from a single primordial parent.">[
[Footnote b: In Bibliothèque Universelle de Genève, Mars, 1860.]
[Footnote c: This we learn from his very interesting article, De la Question de l'Homme Fossile, in the same (March) number of the Bibliothèque Universelle.]
This raises the question, Why does Darwin press his theory to these extreme conclusions? Why do all hypotheses of derivation converge so inevitably to one ultimate point? Having already considered some of the reasons which suggest or support the theory 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 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 duly probabilities to consider. What are these probabilities? What work will this hypothesis do to establish a claim to be adopted in its completeness? Why should a theory which may plausibly enough account for the diversification of the species of each special type or genus, be expanded into a general system for the origination or successive diversification of all species, and all special types or forms, from four or five remote primordial forms, or perhaps 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, for similar reasons; not because it is proved,—thus far it is wholly incapable of proof,—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 the conditions of the case can neither be proved nor disproved by direct evidence or experiment are to be tested only indirectly, and therefore imperfectly, by trying their power to harmonize the known facts, and to account for what is otherwise unaccountable. So the question comes to this:
What will an hypothesis of the derivation of species explain which the opposing view leaves unexplained?