The only instance in which this phenomenon occurs in animals so highly organized as insects, is in the aphides, or plant-lice, In many species of the genus aphis, which in the perfect state, possess wings, a large proportion of the individuals never acquire these organs, but remain in the condition' of Janie. These without ant sexual union (none of them, indeed, being maids) bring forth during the summer living young ones resembling themselves; and these young ones repeat the process, till ten or eleven suc cessive broods are thus produced; the last progeny, toward the end of the summer, being winged males and females, which produce fruitful eggs that retain their vitality during the winter, and give birth to a new generation in the spring, long after their parents have perished. Other peculiarities of insect-generation will be noticed in the article PARTHENOGENESIS.
Several high physiological authorities, amongst whom we may specially mention Huxley (" On the Anatomy of Salpte," in Phil. Trans. for 1851, and " On Animal Indi viduality," in Ann. of lVat. list., 2d ser., vol. ix. p. 303), and Carpenter (Princip(es of Comparative Physiology, 1854), object to the term " alternation of generations." The detached portions of the stock, originating in a single generative act, are termed zooids by these writers, whilst by the term animal or entire animal (the equivalent of coon) they understand in the lower tribes, as in the higher, the collective product of a single gen erative act. Here they include, under the title of one generation, all that intervenes
between one generative act and the next. "If," says Dr. Carpenter, "the phenomena be viewed under this aspect, it will be obvious that the so-called alternation of gene rations' has no real existence; since in every case the whole series of forms which is evolved by continuous development from one generative act repeats itself precisely in the products of the next generative act. The alternation, which is very frequently pre sented in the forms of the lower animals, is between the products of the generative act and the products of gemmation, and the most important difference between them usu ally consists in this—that the former do not contain the generative apparatus which is .evolved in the latter alone. The generating zooid may be merely a segment cast off from the body at large, as in the case of the tape-worms (q.v.), or it may contain a com bination of generative and locomotive organs, as in the self-dividing annelide. It may possess, however, not merely locomotive organs, but a complete nutritive apparatus of its own, which Is thd case in all those instances in which the zooid is cast off in an early stage of its development, and has to attain an increased size, and frequently also to evolve the generative organs, subsequently to its detachment; of this we have exam ples in the medusa budded off from hydroid polypes, and in the aggregate salpos."—Prin of Comparative Physiology, p. 529.