Chap. YL] MODES OF TRANSITION. 233 



early age, before 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 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. 

 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 Mliller, 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. 



