Lastly, this is not the only occasion on which Darwin expressly repudiates Mr. Wallace's opinion on the point in question. For it is notorious that these co-authors of the theory of natural selection have expressed divergent opinions concerning the origin by natural selection of the most general of all specific characters--cross-sterility. Although allowing that cross-sterility between allied species may be of adaptive value in "keeping incipient species from blending," Darwin persistently refused to be influenced by Wallace's belief that it is due to natural selection; i.e. the belief on which alone can be founded the "necessary deduction" with which we have been throughout concerned. NOTE A TO PAGE 57. I think it is desirable here to adduce one or two concrete illustrations of these abstract principles, in order to show how, as a matter of fact, the structure of Weismann's theory is such as to preclude the possibility of its assumptions being disproved--and this even supposing that the theory is false. At first sight nothing could seem more conclusive on the side of Darwinian or Lamarckian principles than are the facts of hereditary disease, in cases where the disease has unquestionably been acquired by the parents. Take, for example, the case of gout. Here there is no suspicion of any microbe being concerned, nor is there any question about the fact of the disease being one which is frequently acquired by certain habits of life. Now, suppose the case of a man who in middle age acquires the gout by these habits of life--such as insufficient exercise, over-sufficient food, and free indulgence in wine. His son inherits the gouty diathesis, and even though the boy may have the fear of gout before his eyes, and consequently avoid over-eating and alcoholic drinking, &c., the disease may overtake him also. Well, the natural explanation of all this is, that the sins of the fathers descend upon the children; that gout acquired may become in the next generation gout transmitted. But, on the other hand, the school of Weismann will maintain that the reason why the parent contracted the gout was because he had a congenital, or "blastogenetic," tendency towards that disease--a tendency which may, indeed, have been intensified by his habits of life, but which, in so far as thus intensified, was not transmitted to his offspring. All that was so transmitted was the congenital tendency; and all that is proved by such cases as those above supposed, where the offspring of gouty parents become gouty notwithstanding their abstemious habits, is that in such offspring the congenital tendency is even more pronounced than it was in their parents, and therefore did not require so much inducement in the way of unguarded living to bring it out. Now, here again, without waiting to consider the relative probabilities of these two opposing explanations, it is enough for the purposes of the illustration to remark that it is