. Elements of comparative zoology. Zoology. 408 GENERAL FIG. 176.—Male (TO) and female (/) of one of the isopod Crustacea, an extreme example of sexual di- morphism. Again, we have to recognize a seasonal dimorphism. Thus certain butterflies produce several broods in a year. Those of the summer broods are so different from those which come from cocoons which have passed through the winter, that without following through the whole history the relationships would not be suspected. Closely connected with this polymorphism is the phe- nomenon of alternation of generations, of which insta
. Elements of comparative zoology. Zoology. 408 GENERAL FIG. 176.—Male (TO) and female (/) of one of the isopod Crustacea, an extreme example of sexual di- morphism. Again, we have to recognize a seasonal dimorphism. Thus certain butterflies produce several broods in a year. Those of the summer broods are so different from those which come from cocoons which have passed through the winter, that without following through the whole history the relationships would not be suspected. Closely connected with this polymorphism is the phe- nomenon of alternation of generations, of which instances are abundant in some groups of the animal kingdom (p. 167). Thus in the butterflies just mentioned, from the eggs of the winter-brood individuals are produced (the summer brood), presenting far different appearances from the parents, while the eggs of the summer brood produce in turn the winter brood. Again, in certain gall-wasps the difference between two generations is so great—both in appearance and in habits—that they would never be regarded as belonging to the same species, or even to the same genus, were it not that the whole his- tory had been followed, so that it was ascertained that each generation resembles, not its parents, but its grand- parents. Another and a more complicated example is furnished by the liver-fluke (p. 180). Many animals in the course of their development pass through a metamorphosis, which is not to be confused with polymorphism. In forms where a metamorphosis occurs the young (the larva), as it hatches from the egg, is greatly different from the parent, but by successive changes of form it at last reaches the adult condition in which it. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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