Having unanimously agreed that organic evolution is a fact and that natural selection is a cause, or a factor in the process, the primary question in debate is whether natural selection is the only cause, or whether it has been assisted by the co-operation of other causes. The school of Weismann maintain that it is the only cause; and therefore deem it worse than useless to search for further causes. With this doctrine Wallace in effect agrees, excepting as regards the particular case of the human mind. The school of Darwin, on the other hand—to which I myself claim to belong—believe that natural selection has been to a considerable extent supplemented by other factors; and, therefore, although we further believe that it has been the "main" factor, we agree with Darwin himself in strongly reprobating all attempts to bar a priori the progress of scientific investigation touching what, if any, these other factors may be. Lastly, there are several more or less struggling schools, chiefly composed of individual members who agree with each other only to the extent of holding that the causal agency of natural selection is not so great as Darwin supposed. The Duke of Argyll, Mr. Mivart and Mr. Geddes may be named in this connexion; together with the self-styled neo-Lamarckians, who seek to magnify the Lamarckian principles at the expense of the distinctively Darwinian.
This primary difference of opinion leads deductively to certain secondary differences. For if a man starts with the premiss that natural selection must necessarily be the "exclusive" cause of organic evolution, he is likely to draw conclusions which another man would not draw who starts with the premiss that natural selection is but the "main" cause. Of these subordinate differences the most important are those which relate to the possible transmission of acquired characters, to the necessary (or only general) utility of specific characters, and to the problem touching the inter-sterility of allied species. But we may well hope that before another ten years shall have passed, even these still outstanding questions will have been finally settled; and thus that within the limits of an ordinary lifetime the theory of organic evolution will have been founded and completed in all its parts, to stand for ever in the world of men as at once the greatest achievement in the history of science, and the most splendid monument of the nineteenth century.
In the later chapters of the foregoing treatise I have sought to indicate certain matters of general principle, which many years of study specially devoted to this great movement of contemporary thought have led me to regard as almost certainly sound in themselves, and no less certainly requisite as complements of the Darwinian theory. I will now conclude by briefly summarizing these matters of general principle in the form of twelve sequent propositions. And, in doing so, I may ask it to be noticed that the system which these propositions serve to express may now claim, at the least, to be a strictly logical system. For the fact that, not merely in its main outlines, but likewise in its details, it has been independently constructed by Mr. Gulick, proves at any rate this much; seeing that, where matters of such intricacy are concerned, nothing but accurate reasoning from a common foundation of data could possibly have yielded so exact an agreement. The only difference between us is, that Mr. Gulick has gone into much further detail than I have ever attempted in the way of classifying the many and varied forms of isolation; while I have laid more special stress upon the physiological form, and found in it what appears to me a satisfactory solution of "the greatest of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species"—namely, "the remarkable difference between varieties and species when crossed."
GENERAL CONCLUSIONS.
1. Natural Selection is primarily a theory of the cumulative development of adaptations wherever these occur; and therefore is only incidentally, or likewise, a theory of the origin of species in cases where allied species differ from one another in respect of peculiar characters, which are also adaptive characters.
2. Hence, it does not follow from the theory of natural selection that all species—much less all specific characters—must necessarily have owed their origin to natural selection; since it cannot be proved deductively from the theory that no "means of modification" other than natural selection is competent to produce such slight degrees of modification as go to constitute diagnostic distinctions between closely allied species; while, on the other hand, there is an overwhelming mass of evidence to prove the origin of "a large proportional number of specific characters" by causes of modification other than natural selection.
3. Therefore, and upon the whole, as Darwin so emphatically held, "Natural selection has been the main, but not the exclusive means of modification."
4. Even if it were true that all species and all specific characters must necessarily owe their origin to natural selection, it would still remain illogical to define the theory of natural selection as indifferently a theory of species or a theory of adaptations; for, even upon this erroneous supposition, specific characters and adaptive characters would remain very far indeed from being conterminous—most of the more important adaptations which occur in organic nature being the common property of many species.