the conditions have not changed. The degeneration sets in at that period when the ancestral use is persistently denied. There is no reason why "disuse" should in all cases remove all remnants of a structure; but if the presence of the degenerate tissue is a source of danger to the organism which possesses it, that organism will be eliminated, and those (1) which possess it in an inert, harmless form, or (2) in which it is absent, will survive. Thus natural selection (which will fall under Mr. Romanes's reversed selection) will step in--will in some cases reduce the organ to a harmless and degenerate rudiment, and in others remove the last vestiges of the organ. On the whole, even taking into consideration the effects of panmixia, of reversed selection, and of the principle of economy, the reduction of organs is difficult to explain, unless we call into play "disuse" as a co-operating factor. _Sexual Selection, or Preferential Mating._ It is well known that, in addition to and apart from the primary sexual differences in animals, there are certain secondary characters by which the males, or occasionally the females, are conspicuous. The antlers of stags, the tail of the peacock, the splendid plumes of the male bird of paradise, the horns or pouches of lizards, the brilliant frilled crest of the newt, the gay colours of male sticklebacks, the metallic hues of male butterflies, and the large horns or antennæ of other insects,--these and many other examples which will at once occur to the reader are illustrations of the fact.