MODES OF TRANSITION. 177 



lacnnagof the sack and body, and which have been consid- 

 ered by all naturalists to act as branchiae. Now I think no 

 one will dispute that the ovigerons frena in tlie one family 

 are strictly homologous with the branchiEe of the other 

 family; indeed, they graduate into each other. Therefore 

 it need not bo doubted that the two little folds of skin, 

 which originally served as ovigerons frena, but which, like- 

 wise, very slightly aided in the act of respiration, have 

 been gradually converted by natural selection into branchise, 

 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 extinc- 

 tion than have sessile cirripedes, who would ever have im- 

 agined that the branchise in this latter family had origin- 

 ally 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 Professor 

 Cope and others in the United States. It is now known 

 that some animals are capable of reproduction at a very 

 early age, before they have acquired their perfect charac- 

 ters; and if this power became thoroughly well developed 

 in a species, it seems probable that the adult stage of devel- 

 opment 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 ai'iiving at 

 maturity, go on changing in character 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. Professor Cope states that 

 the teeth of certain lizards change much in shape with ad- 

 vancing years. With crustaceans not only many trivial, 

 but some important parts assume a new character, as re- 

 corded by Fritz Miiller, after maturity. In all such cases 

 ■ — and many could be given — if the age for reproduction 

 were retarded, the character of the species, at least in its 

 adult state, would be modified; nor is it improbable that 



