EEGENERATION 15 



particular part of the bocl}^, while the rest of the very important 

 but rarely injured parts do not possess it at all, again points to 

 the conclusion that the faculty of regeneration has an adaptive 

 character. 



It does not affect matters to discover cases in which we cannot 

 recognize this relation between the regenerative capacity of a part 

 and its importance or its liability to injury. Such instances do not 

 lessen the convincingness of the positive cases, since we do not know 

 the exact conditions which may lead to the increase of regenerative 

 capacit}^ in a part, and, above all, since we do not know^ the rate at 

 which such an increase may take place. If adaptation in general 

 depends upon processes of selection, these processes must also be able 

 to give rise to an increase in the power of regeneration. On the 

 other hand, it by no means follows that the disappearance of a faculty 

 of regeneration which was once present in a part, but which has 

 become superfluous in the course of time, must take place immediately 

 through natural selection. For it is the very essence of natural 

 selection that it only furthers what is useful, and only removes what 

 is injurious ; over what is indifferent it has no power at all. Thus it 

 follows that the faculty of regeneration, when it has once been present 

 in a part, cannot be set aside by natural selection (personal selection), 

 for it is in no way injurious to its possessor. If it graduall}^ decreases 

 and becomes extinct notwithstanding this, when it is of no further 

 use, as seems to be to some extent the case in regard to the legs and 

 tail of the blind Proteus, that must depend on other processes, 

 on those which generally bring about the gradual disappearance 

 of disused parts or capacities. We shall attempt to probe to the 

 roots of these processes later on ; for the present let it suffice us to 

 know that, according to our exi^erience, they go on with exceeding 

 slowness, and that it has taken whole geological periods to eliminate 

 the legs of the snake-ancestors so complete^ as has been done from 

 the structure of most of our modern snakes, while the Proteus which 

 migrated into the caves of Krain as far back as the Cretaceous period 

 is indeed blind, but still retains its eyes under the skin, tliough in 

 a degenerate condition. 



Since the degeneration of disused parts and capacities goes on so 

 slowly it need not surprise us that we meet man}^ parts which still 

 possess regenerative capacity, although they are protected from injury. 

 Thus Morgan found that, in the hermit-crab, the limbs which are 

 protected within the mollusc shell were quite as ready to regrow as 

 those which are actually used for walking, and thus are exposed to 

 possibility of attack, but this proves nothing against the conclusion 



