Such species, imperfect because of their "disharmonies," are an illustration of the fact that Nature herself, in matters relating to living things, is not averse to compromise. Nature sets the example of toleration. Toleration may be defended on the ground that it is the biological method. Nature, though stern and inexorable as to essentials, yet accepts the faults and defects of some of her children because of the virtues and excellences which accompany them. The most highly endowed and successful forms on account of their dominance and power of spreading into new conditions, are even more likely than less highly developed kinds to retain concealed defects--disharmonies which do not lead to the destruction of the species, but occasionally cause strange embarrassment to it until they are, possibly in the long process of ages, got rid of by the slow operation of selection and survival of those individuals in which the injurious character varies in the direction of diminution and ultimate disappearance.
In man (owing, apparently, to the rapid rate at which he has been carried along towards dominance over the whole face of the globe by the development of his intelligence) the bodily structure has failed to keep pace with and to become perfected, "trimmed up," and completely adapted to, the newly-acquired habits which his increasing intelligence has forced on him. His "wisdom teeth" are "disharmonies." They are now useless and dwindled, weak spots open to the attacks of disease--since they are no longer needed for grinding coarse vegetable food, and are consequently no longer kept (by the speedy death of those individuals in whom they are small) at the full original size and efficiency seen in the apes. His large intestine is a "disharmony" not yet got rid of by natural selection, although no longer useful, but, on the contrary, the seat of poisonous putrefaction and absorption of such poisons. His tail--a few small vertebræ beneath the skin--is absolutely useless, and occasionally the seat of dangerous injury or disease. Tails very generally are liable to become useless in the descendants of animals in which they were invaluable as "fly-brushes" (cattle, horses, etc.), as prehensible organs (American monkeys), as concealing cloaks (South American ant-eater), as aids to swimming or flying, or as ornamental glories (the big-cats and others). The stumpy tail of the lynx, of some monkeys, and some lizards and fishes tells of a history in which the full-sized tail became a "disharmony"--a positive nuisance--and has been reduced, even if not abolished, by natural selection of short-tailed or tail-less varieties.