356 Evolution and Adaptation 



the new mutation may persist. We need not suppose that 

 the original form becomes crowded out, but only that a more 

 degenerate form has come into existence. As a matter of 

 fact we find in most groups, in which degenerate forms exist, 

 a number of different stages in the degeneration in different 

 species. Mutation after mutation might follow until many of 

 the original organs have disappeared. The connection that 

 appears to exist between the degeneration of a special part 

 and the environment in which the animal lives finds its 

 explanation simply in the fact that the environment makes 

 possible the existence of that sort of mutation in it. We do 

 not know, as yet, whether through mutative changes an 

 organ can completely disappear, although this seems probable 

 from the fact that in a few cases mutations are known to 

 have arisen in which a given part is entirely functionless. 

 If we could assume that, a mutation in the direction of 

 degeneration being once established, further mutations in 

 the same direction would probably occur, the problem 

 would be much simplified ; but we lack data, at present, to 

 establish this view. 



In the case of blind animals it seems probable that the 

 transition has taken place in such forms as had already- 

 established themselves in places more or less removed from 

 the light. Such forms as had the habit of hiding away under 

 stones, or in the ground, living partly in and partly out of 

 the light, might, if a mutation appeared of such a sort that 

 amongst other changes the eyes were less developed, still be 

 capable of leading an existence in the dark, while it might be 

 impossible for them to exist any longer with weakened vision 

 in the light. If such a process took place, the habitat of the 

 new form would be limited, or in other words it would be 

 confined to the locality to which it finds itself adapted ; 

 not that it has become adapted to the environment through 

 competition with the original species, or, in fact, with any 

 other. 



