Chap. VI.] MODES OF TRANSITION. 033 



early age, before they have acquired their perfect charac- 

 ters; and if this power became thoroughly well de- 

 veloped 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 in- 

 stances 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 Miil- 

 ler, 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 

 often or ever been modified through this comparatively 

 sudden mode of transition, I can form no opinion; but 

 if this has occurred, it is probable that the differences 

 between the young and the mature, and between the 

 mature and the old, were primordially acquired by 

 graduated steps. 



