MODES OF TRANSITION. 167 



reproduction. 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 

 iegraded. 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. Professor 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 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 

 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. 



SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL 



SELECTION. 



Although we must be extremely cautious in concluding 

 that any organ could not have been produced by successive, 

 small, transitional gradations, yet undoubtedly serious cases 

 of difficulty occur. 



One of the most serious is that of nenter insects, which 

 are often differently constructed from either the males or 

 fertile females ; but this case will be treated of in the next 

 chapter. The electric organs of fishes offer another case 



