. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . y animal at any stage in its existence differs absolutely fromany other kind of animal, in that it can develop into only itsown kind of animal. There is something inherent in eachdeveloping animal that gives it an identity of its own. Al-though in its young stages it may be hardly distinguishablefrom some other kind of animal in similar stages, it is sure tocome out, when fully developed, an individual of the samekind as its parents were or are. A very yo
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . y animal at any stage in its existence differs absolutely fromany other kind of animal, in that it can develop into only itsown kind of animal. There is something inherent in eachdeveloping animal that gives it an identity of its own. Al-though in its young stages it may be hardly distinguishablefrom some other kind of animal in similar stages, it is sure tocome out, when fully developed, an individual of the samekind as its parents were or are. A very young fish and a veryyoung salamander are almost indistinguishably alike, but oneis sure to develop into a fish and the other into a certainty of an embryo to become an individual of acertain kind is called the law of heredity. Viewred in the lightof development, there must be as great a difference betweenone egg and another as between one animal and another, forthe greater difference is included in the less. The significance of the developmental phenomena is amatter about which naturalists have yet very much to learn. c. FIG. 135.—Stages in the development of the prawn, Peneus potimiriitm: A, Naupliuslarva; B, first zoea stage; C, second zoea stage. (After Fritz Miiller.) larva; B, first zoea stage; C, second zoea stage It is believed, however, by practically all naturalists that manyof the various stages in the development of an animal cor-respond to or repeat, in many fundamental features at least,the structural condition of the animals ancestors. Naturalistsbelieve that all backboned or vertebrate animals are relatedto each other through being descended from a common ancestor,the first or oldest backboned animal. In fact, it is because all GENERATION, SEX AND ONTOGENY 233 these backboned animals--the fishes, the batrachians, thereptiles, the birds, and the mammals—have descended froma common ancestor that they all have a backbone. It isbelieved that the de
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