316 



HABIT AND INTELLIGENCE. 



[chap. 



Origin of 

 the bat's 

 wing. 



Self-adap- 

 tation and 

 natural 

 selection 

 co-ope- 

 rating. 



Extensor 

 muscle of 

 wing of 

 flying 

 lemur. 



after generation, of squirrels that had the fullest skin on 

 their flanks, and were thus best able to escape their enemies 

 by taking long leaps, is quite an adequate cause for the 

 orighi of such a structure as the membrane of the flying 

 squirrel. Further development into such a perfect flying 

 apparatus as that of the bat is a much greater change 

 than that from the common to the flying squirrel ; but 

 self-adaptation will account for this in a great degree ; 

 once the animal began to gain anything by flapping its 

 membranes, the muscles used in doing so would strengthen 

 and grow, in virtue of the law that eveiy muscle strengthens 

 and grows with use. There is no difficulty in believing 

 that it would begin to flap its membranes as soon as it 

 became useful to do so ; if mere animal intelligence were 

 not sufficient to originate such an instinct, it might begin 

 in some spontaneous variation, much less strange than that 

 in which the characteristic habit of the tumbler pigeon 

 began. This is exactly the same as the question how 

 animals with legs first learned to creep and to walk. 



I agree with Darwin in believing that natural selection 

 can produce changes which self-adaptation has no ten- 

 dency to produce. But the process of self-adaptation, in 

 whatever direction it is going on, will be so furthered and 

 assisted by natural selection that the effects of the two 

 will be impossible to separate. Self-adaptation gradually 

 adapts organisms to their mode of life ; natural selection 

 destroys those which are least adapted ; and it is obvious 

 that when these factors are both in operation, they will 

 work to the same result. 



But is this explanation of the probable origin of the 

 bat's or the flying lemur's wing satisfactory ? Does it 

 explain all the facts ? I greatly doubt whether it does. 1 

 do not see how it will account for the fact that in the 

 flying lemur " the flank membrane is also furnished with 

 an extensor muscle."^ I do not think that any clear 

 evidence has been brought, proving that either self- adapta- 

 tion or spontaneous variation wUl suffice to produce a 

 new muscle. But there are so many instances of organs 

 1 See quotation from Darwin above. 



