MODES OF TRANSITION. I77 



lacunae of the sack and body, and which have been consid- 

 ered by all naturalists to act as branchiae. Now 1 tliink no 

 one will dispute that the ovigerous frena in the one family 

 are strictly homologous with the branchiae of the other 

 family; indeed, they graduate into each other. Tlierefore 

 it need not be doubled that the two little folds of skin, 

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

 wise, 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 obliteratii^n 

 of their adhesive glands. If all pedunculated cirri pedes 

 had become extinct, and they have suffered far more extinc- 

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

 agined that the branchiae in this latter family had origin- 

 ally existed as organs for preventing the ova from being 

 washed out of the sack? 



There is anotlier 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 arriving 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 aiul moi-e 

 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 shajie 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 



