DIFFICULTIES OF THE THEORY 259 



the branchi83 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 

 reproduction. 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, before they have acquired their perfect char- 

 acters; and if this power became thoroughly well devel- 

 oped 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 ani- 

 mals, 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 retarded, 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 

 hurried through and finally lost. Whether species have 



