Seite - 197 - in The Origin of Species

MODES OF TRANSITION 197 the one family are strictly homologous with the branchiae of the other family ; indeed, they graduate into each other. Therefore it need not be doubted that the two little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided in the act of respiration, have been gradually converted by natural selection into branchiae, simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated cirripedes had become extinct, and they have suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the sack ? There is another possible mode of transition, namely, through the acceleration or retardation of the period of re- production. This has lately been insisted on by Prof. Cope and others in the United States. It is now known that some animals are capable of reproduction at a very early age, be- fore they have acquired their perfect characters; and if this power became thoroughly well developed in a species, it seems probable that the adult stage of development would sooner or later be lost; and in this case, especially if the larva differed much from the mature form, the character of the species would be greatly changed and degraded. Again, not a few animals, after arriving at maturity, go on changing in char- acter during nearly their whole lives. With mammals, for instance, the form of the skull is often much altered with age, of which Dr. Murie has given some striking instances with seals ; every one knows how the horns of stags become more and more branched, and the plumes of some birds become more finely developed, astheygrow older. Prof. Cope states that the teeth of certain lizards change much in shape with advancing years ; with crustaceans not only many trivial, but some important parts assume a new character, as recorded by Fritz Miiller, after maturity. In all such cases,—and many could be given,—if the age for reproduction were re- tarded, the character of the species, at least in its adult state, would be modified ; nor is it improbable that the previous and earlier stages of development would in some cases be burned through and finally lost. Whether species have often or ever