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. 
