It has often been pointed out that certain organs may be more perfectly developed than the requirements of the surroundings strictly demand. At least we have no good reasons to suppose in some cases that constant selection is keeping certain organs at the highest possible point of development, yet, on the Darwinian theory, as soon as selection ceases to be operative the level of perfection must sink to that which the exigencies of the situation demand. The problem may be expressed in a different way. Does the animal or plant ever possess organs that are more perfectly adapted than the absolute requirements demand? If such organs are the result of fluctuating variations, they will be unable to maintain themselves in subsequent generations without a constant process of selection going on. If, on the other hand, the organs have arisen as mutations, they may become permanently established without respect to the degree of perfection of their adaptation. We can see, therefore, that cases of extreme perfection meet with no difficulty on the mutation theory, while they have proven one of the stumbling-blocks to the selection theory. There are, in fact, many structures in the animal and plant kingdoms that appear to be more perfect than the requirements seem to demand. The exact symmetry of many forms appears in some cases to be unnecessarily perfect. The perfection of the hand of man, the development of his vocal organs, and certain qualities of his brain, as his musical and mathematical powers, seem to go beyond the required limits. It is not, of course, that these things may not be of some use, but that their development appears to have gone beyond what selection requires of these parts. Closely related to this group of phenomena are those cases in which certain organs are well developed, but which can scarcely be of use to the animal in proportion to their elaboration. The electric organs of several fishes and skates are excellent examples of this sort of structures. The phosphorescent organs do not appear, in some forms at least, to be useful in proportion to their development. The selection theory fails completely to explain the building up of organs of this kind, but on the mutation theory there is no difficulty at all in accounting for the presence of even highly developed organs that are of little or of no use to the individual. If the organs appeared in the first place as mutations, and their presence was not injurious to the extent of interfering seriously with the existence and propagation of the new form, this new form may remain in existence, and if the mutations continued in the same direction, the organs might become more perfect, and highly developed. The whole class of secondary sexual organs may belong to this category, but a discussion of these organs will be deferred to the following section. Secondary Sexual Organs as Adaptations