6. At this point, therefore, it may well seem that we need not have troubled ourselves any further with a generalization which does not appear to have anything to support it. And to this view of the case I should myself agree, were it not that many naturalists now entertain the doctrine as an essential article of their Darwinian creed. Hence, I proceeded to adduce considerations _per contra_.
Seeing that the doctrine in question can only rest on the assumption that there is no cause other than natural selection which is capable of originating any single species--if not even so much as any single specific character--I began by examining this assumption. It was shown first that, on merely antecedent grounds, the assumption is "infinitely precarious." There is absolutely no justification for the statement that in all the varied and complex processes of organic nature natural selection is the only possible cause of specific change. But, apart altogether from this _a priori_ refutation of the dogma, our analysis went on to show that, in point of actual fact, there are not a few well-known causes of high generality, which, while having no connexion with the principle of utility, are demonstrably capable of originating species and specific characters--if by "species" and "specific characters" we are to understand organic types which are ranked as species, and characters which are described as diagnostic of species. Such causes I grouped under five different headings, viz. Climate, Food, Sexual Selection, Isolation, and Laws of Growth. Sexual Selection and Isolation are, indeed, repudiated by Mr. Wallace; but, in common I believe with all biologists, he accepts the other three groups of causes as fully adequate to produce such kinds and degrees of modification as are taken to constitute specific distinction. And this is amply sufficient for our present purposes. Besides, under the head of Sexual Selection, it does not signify in the present connexion whether or not we accept Darwin's theory on this subject. For, in any case, the facts of secondary sexual characters are indisputable: these characters are, for the most part, specific characters: and they cannot be explained by the principle of utility. Even Mr. Wallace does not attempt to do so; and the explanation which he does give is clearly incompatible with his doctrine touching the necessarily life-serving value of all specific characters. Lastly, the same has to be said of the Laws of Growth. For we have just seen that on the grounds of this principle likewise Mr. Wallace abandons the doctrine in question. As regards Isolation, much more remains to be said in the ensuing portion of this work, while, as regards Climatic Variation, there are literally innumerable cases where changes of specific type are known to have been caused by this means.