Now it is to be remarked that neither in these passages nor in any of the other less distinct expressions of opinion on this question, does Darwin ever admit that "specific characters"--that is, the particular characters which serve to distinguish one species from another--are ever useless, much less that "a large proportion of them" are so, as Mr. Romanes makes him "freely acknowledge." On the other hand, in the passage which I have italicised he strongly expresses his view that much of what we suppose to be useless is due to our ignorance; and as I hold myself that, as regards many of the supposed useless characters, this is the true explanation, it may be well to give a brief sketch of the progress of knowledge in transferring characters from the one category to the other[151]."
It is needless to continue this quotation, because of course no one is disputing that an enormous number of specific characters whose utility is unknown are nevertheless useful, and therefore due to natural selection. In other words, the question is not--Are there not many useful specific characters whose utility is unknown? but--Does it follow from the theory of natural selection that all specific characters must necessarily be useful? Well, it appears to me that without going further than the above passage, which Mr. Wallace has quoted, we can see clearly enough what was Darwin's opinion upon the subject. He did not believe that it followed _deductively_ from his theory that all specific characters must necessarily be useful; and therefore he regarded it as a question of _fact_--to be determined by induction as distinguished from deduction--in what proportional number of cases they are so. Moreover he gives it as his more matured opinion, that, "as far as we can at present judge" (i.e. from the present state of observation upon the subject: if, with Mr. Wallace, his judgement were _a priori_, why this qualification?), he had not previously sufficiently considered the existence of non-adaptive characters--and this he ended by believing was one of the greatest oversights as yet detected in his work. To me it has always seemed that this passage is one of the greatest exhibitions of candour, combined with solidity of judgement, that is to be met with even in the writings of Darwin. There is no talk about any deductive "necessity"; but a perfect readiness to allow that causes other than natural selection may have been at work in evoking non-adaptive characters, so that the fifth edition of the _Origin of Species_ was altered in order to confine the theory of natural selection to "adaptive changes"--i.e. to constitute it, as I have said in other words, "a theory of the origin, or cumulative development, of _adaptations_."