Ι II II aga Salamander Tortoise Chick III Rabbit FIG. 41.-Different vertebrate animal in successive embryonic stages 1, first or earliest of the stages figured; II, second of the stages; III, third or latest of the stages.-After HAECKEL. hopper goes through the same developmental changes from single egg cell to the full-grown active hopper as every other grasshopper of the same kind-that is, development takes place according to certain natural laws, the laws of animal development. These laws may be roughly stated as follows: All many-celled animals begin life as a single cell, the fertilized egg cell; each animal goes through a certain orderly series of developmental changes which, accompanied by growth, leads the animal to change from single cell to the many-celled, complex form characteristic of the species to which the animal belongs; this development is from simple to complex structural condition; the development is the same for all individuals of one species. While all animals begin development similarly, the course of development in the different groups soon diverges, the divergence being of the nature of a branching, like that shown in the growth of a tree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed condition, all standing clearly apart from each other. But in tracing back the development of any kind of animal, we soon come to a point where it very much resembles or becomes apparently identical with some other kind of animal, and going further back we find it resembling other animals in their young condition, and so on until we come to that first stage of development, that trunk stage, where all animals are structurally alike. To be sure, any animal at any stage in its existence differs absolutely from any other kind of animal, in that it can develop into only its own kind of animal. There is something inherent in each developing animal that gives it an identity of its own. Although in its young stages it may be indistinguishable from some other kind of animal in similar stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. The young fish and the young salamander in the upper row in Fig. 41 are indistinguishably alike, but one embryo is sure to develop into a fish and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law of heredity. Viewed in the light of development, there must be as great a difference between one egg and another as between one animal and another, for the greater difference is included in the less. 55. The significance of the facts of development.-The significance of the developmental phenomena is a matter about which naturalists have yet very much to learn. It is believed, however, by practically all naturalists that many of the various stages in the development of an animal correspond to or repeat the structural condition of the animal's ancestors. Naturalists believe that all backboned or vertebrate animals are related to each other through being descended from a common ancestor, the first or oldest backboned animal. In fact, it is because all these backboned animals-the fishes, the batrachians, the reptiles, the birds, and the mammals-have descended from a common ancestor that they all have a backbone. It is believed that the descendants of the first backboned animal have in the course of many generations branched off little by little. from the original type until there came to exist very real and obvious differences among the backboned animals-differences which among the living backboned animals are familiar to all of us. The course of development of an individual animal is believed to be a very rapid and evidently much condensed and changed, recapitulation of the history which the species or kind of animal to which the developing individual belongs has passed through in the course of its descent through a long series of gradually changing ancestors. If this is true, then we can readily understand why the fish and the salamander and tortoise and bird and rabbit are all so much alike in their earlier stages of development, and gradually come to differ more and more as they pass through later and later developmental stages. 56. Metamorphosis.-While a young robin when it hatches from the egg or a young kitten at birth resembles its parents, a young star-fish or a young crab or a young butterfly when hatched does not at all resemble its parents. And while the young robin after hatching becomes a fully grown robin simply by growing larger and undergoing comparatively slight developmental changes, the young star-fish or young butterfly not only grows larger, but undergoes some very striking developmental changes; the body changes very much in appearance. Marked changes in the body of an animal during post-embryonic or larval development constitute what is called metamorphic development, or the animal is said to undergo or to show metamorphosis in its development. Metamorphosis is one of the most interesting features in the life history or development of animals, and it can be, at least as far as its external aspects are concerned, very readily observed and studied. 57. Metamorphosis among insects.-All the butterflies and moths show metamorphosis in their development. So do many other insects, as the ants, bees, and wasps, and all the flies and beetles. On the other hand, many insects do not show metamorphosis, but, like the birds, are hatched from the egg in a condition plainly resembling the parents. A grasshopper (Fig. 42) is a convenient example of an insect without metamorphosis, or rather, as there are, after all, a few easily perceived changes in its post-embryonic development, of an insect with an "incomplete metamorphosis." The eggs of grasshoppers are laid in little packets of several score half an inch below the surface of the ground. When the young grasshopper hatches from the egg it is of course very small, but it is plainly recognizable as a grasshopper. But in one important character it differs from the adult, and that is in its lack of wings. The adult grasshopper has two pairs of wings; the just hatched young or larval grasshopper has no wings at all. The young grasshopper feeds voraciously and grows rapidly. In a few days it molts, or casts its outer skin (not the true skin, but a thin, firm covering or outer body wall composed of a substance called chitin, which is secreted by the cells of the true skin). In this second larval stage there can be seen the rudiments of four wings, in the condition FIG. 42.-Post-embryonic development (incomplete metamorphosis) of the Rocky Mountain locust (Melanoplus spretus). a, b, c, d, e, and f, successive developmental stages from just hatched to adult individual.-After EMERTON. of tiny wing pads on the back of the middle part of the body (the thorax). Soon the chitinous body covering is shed again, and after this molt the wing pads are markedly larger than before. Still another molt occurs, with another increase in size of the developing wings, and after a fifth and last molt the wings are fully developed, and |