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, as they grow 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 



