alike in all cases. Before answering this question, we must call the attention of the unscientific reader to a familiar fact which will throw great light upon the matter.
The animals with which we are most familiar, the mammals and birds, are born in substantially the form which they will have when they reach maturity. They breathe the same medium; they employ the same organs of locomotion, in the same way; they require the same or nearly the same kind of food, and their habits and surroundings are the same as they will be during mature life, or at least the differences are slight and insignificant, and the adult is little more than the young animal grown to its full size, and with sexual characteristics, and able to reproduce its kind. But we must recollect that, in the greater part of the animal kingdom, this is not the case. In by far the greater part of the species of animals the rule is that the newly born young is very different, in structure, in habits, surroundings, and needs, from the adult, and its passage to the adult form is not simply a process of growth, but a process of great change in every particular.
The young butterfly crawls over the plant on which it is born, and finds an abundant supply of proper food in the green leaves, which it cuts to pieces with its strong, scissor-like jaws. Its capacious digestive tract is fitted for dealing with great quantities of bulky but very slightly nutritious food; and its enemies, dangers, and means of defense are very different from those of the adult winged insect, which is furnished with highly developed sense-organs, and flies from place to place in search of the highly concentrated liquid food adapted for sucking up through the proboscis which has replaced the cutting jaws of the young; and we must recollect that the life-history of the butterfly, so far as its great changes are concerned, is a type of the life of numbers of other animals, for nearly all the invertebrates pass through a metamorphosis.
Whenever young animals are left to shift for themselves, without parental protection, they are compelled to struggle for existence with a host of competitors and enemies; and in all cases where the structure and habits of the young differ from those of the adult, a variation in the young animal may be as important for the welfare of the species as one in the adult, and may, therefore, be seized upon and perpetuated by natural selection, and in this way the young stages of two closely related species may be modified in different directions until they become quite different from each other, while the adults may remain essentially alike; and as natural selection may act in such a way as to modify the life-history of an organism at any stage of its existence, there is no limit to the secondary changes which may thus be brought about. A larva may acquire new organs, or it may lose old ones; the order in which organs are acquired may be modified; stages of development may be dropped out of the series, or new ones