have already dealt, and to which therefore we need not devote much space. He writes—
"For now observe the fact which here chiefly concerns us, that the survival of the fittest can increase any serviceable trait, only if that trait conduces to prosperity of the individual, or of posterity, or of both, in an important degree. There can be no increase of any structure by natural selection unless, amid all the slightly varying structures constituting the organism, increase of this particular one is so advantageous as to cause greater multiplication of the family in which it arises than of other families. Variations which, though advantageous, fail to do this must disappear again. Let us take a case" (pp. 11–2).
He then proceeds to discuss the numerous qualities which in a herd of deer conduce to survival—keenness of scent, sight, and hearing, excellence of digestion through superiority of teeth, gastric juice, stomach, &c., agility, sagacity, power of resisting flies, cold, and so forth. Founding his argument apparently on the assumption that an individual of the herd which is superior as regards one quality will be inferior as regards the others, he comes to the conclusion—"If these other individuals severally profit by their small superiorities, and transmit them to equally large numbers of descendants, no increase of the variation in question can take place, it must soon be cancelled" (p. 13). In other words, interbreeding must soon swamp the specified variation.
The fallacy that underlies Mr. Spencer's argument is this. Excellence in one quality is not usually found dissociated with excellence in other qualities. The vigorous in one respect may be, and commonly are, vigorous in other respects also. Moreover, the qualities are not isolated—keenness of sight in one deer, keenness of
