NATURE AND NURTURE 57 



sighted than impatience with the restlessness of children, although 

 no effeminacy can wholly repress the joyous exuberancy of child- 

 hood ; nor can any thoughtful person fail to see that the impulse 

 which leads young animals to train and develop their bodies by 

 sports and gambols is adaptive. 



All this, and more, is implied by the admission that there is 

 such a thing as nurture; and one of the first questions to present 

 itself, when we consider the matter, is why living things are not 

 like the imaginary Caliban ; how they come by a nature on which 

 nurture will stick ; for it is plain that, far from being an explana- 

 tion of nature, nurture is a fact which itself calls for explanation. 



The most stable organs may be modified by novel or excep- 

 tional use, and the most profound structural changes may be 

 brought about by nurture. After Hunter had fed a sea-gull on 

 grain for a year, he found that the inner coat of its stomach had 

 grown hard, and its muscles had thickened, thus forming a true 

 gizzard, although the sea-gull normally has a soft stomach, as it 

 lives upon the soft flesh of fishes. It is well known that living 

 things are often changed by mechanical influences. The skull 

 of a hornless ram has been found to weigh only one-fourth as 

 much as the skull of a ram with horns ; and the whole configu- 

 ration of the skull of lop-eared rabbits is altered by the mechanical 

 pressure of the drooping ears. Hemp seed causes bulfinches and 

 some other birds to become black ; and we know, from the obser- 

 vations of many naturalists, that change of food sometimes changes 

 the colors of caterpillars, or even those of the moths which they 

 produce. Many curious cases of this sort have been recorded, in 

 birds and insects, and it seems reasonable to believe that, if un- 

 natural food may change the normal colors of a species, the normal 

 colors may themselves, in some cases, be due to the direct action 

 of the natural food. 



Sometimes the effect of the conditions of life is injurious, some- 

 times neutral, but often it is useful to a notable degree ; and it is 

 this usefulness — the power to respond to changed conditions by 

 adaptive modification — which is most worthy of consideration. 

 Cold weather promotes the growth of hair on mammals, and thus 

 protects them from the cold. The muscle which is used grows 

 stronger, and the hand becomes skilful by training. 



