In the instance of the male and female insect just noticed, the male is active, and has to search for his partner, and the female is a stay-at-home, and expects to be courted, and when mated to do nothing more than lay eggs. Her wings would be of doubtful value. We may believe, then, that _disuse_, generation after generation, gradually weakened the wing, and finally Nature, ever economical in not-used organs, did not perpetuate it. Disuse may be therefore considered as the principal cause of the atrophy, rudimentary condition, and of the final deficiency of structures. But disuse will not produce this in one generation, but in many, so it is necessary to look farther back into the ancestry of the creatures which have rudimentary organs. The four-legged ruminating or cud-chewing animals have bones and feet of peculiar arrangement, and there is no difficulty in at once knowing a ruminant by its bones. Now, in former ages, and before there was a trace of man on the globe, there were ruminants, as known by their bones found in strata or deposits, and they had incisor teeth in their upper jaws when full grown, and not only when in the calf condition. The inference to be drawn is, that the modern Oxen are the descendants of those ancient forms with incisor teeth, and that disuse, probably produced by the introduction of grass-feeding on a grand scale, instead of leaf-and bud-nibbling, gradually diminished the strength and permanence of the front upper teeth, and finally only left the simple traces of them which we have mentioned. Disuse by ancestral forms, by the forefathers, and the carrying down the weakened and atrophied state of the structure or organs, are the most important considerations in any attempt at the explanation of the seeming paradox. In endeavouring to apply this style of reasoning to the Colobos group--the Semnopitheci without thumbs--it must be asked, is there any evidence of the great antiquity of these Monkeys, and are there any evidences of anything wrong about the thumbs of their Asiatic allies? [Illustration: GUEREZA.]