Seite - 479 - in The Origin of Species

DEVELOPMENT AND EMBRYOLOGY 479 of the Cecidomyia with the parthenogenesis of the Coc- cidae;"—the term parthenogenesis implying that the mature females of the Coccida; are capable of producing fertile eggs without the concourse of the male. Certain animals belong- ing to several classes are now known to have the power ot ordinary reproduction at an unusually early age ; and we have only to accelerate parthenogenetic reproduction by gradual steps to an earlier and earlier age,—Chironomus showing us an almost exactly intermediate stage, viz., that of the pupa—and we can perhaps account for the marvellous case of the Cecidomyia. It has already been stated that various parts in the same individual which are exactly alike during an early embryonic period, become widely different and serve for widely dilifer- ent purposes in the adult state. So again it has been shown that generally the embryos of the most distinct species be- longing to the same class are closely similar, but become, when fully developed, widely dissimilar. A better proof oi this latter fact cannot be given than the statement of Von Baer that "the embryos of mammalia, of birds, lizards, and "snakes, probably also of chelonia, are in their earliest states "exceedingly like one another, both as a whole and in the "mode of development of their parts; so much so, in fact, "that we can often distinguish the embryos only by their "size. In my possession are two little embryos in spirit, "whose names I have omitted to attach, and at present I am "quite unable to say to what class they belong. They may "be lizards or small birds, or very young mammalia, so "complete is the similarity in the mode of formation of the "head and trunk in these animals. The extremities, however, "are still absent in these embryos. But even if they had "existed in the earliest stage of their development we should "learn nothing, for the feet of lizards and mammals, the "wings and feet of birds, no less than the hands and feet "of man, all arise from the same fundamental form." The larvae of most crustaceans, at corresponding stages of devel- opment, closely resemble each other, however different the adults may become; and so it is with very many other ani- mals. A trace of the law of embryonic resemblance occa- sionally lasts till a rather late age : thus birds of the same