228 TnE INDUCTIONS OF BIOLOGY. 



organism as a whole retains pretty nearly the same bulk, the 

 proportions of its parts may be considerably varied. Their 

 variations, here treated of under the title Adaptation, depend 

 on specialities of individual action. In the last chapter we 

 saw that the actions of organisms entail re-actions on them; 

 and that specialities of action entail specialities of re-action. 

 Here it remains to be pointed out that these special actions 

 and re-actions do not end with temporary changes, but work 

 permanent changes. 



If, in an adult animal, the waste and repair in all parts 

 were exactly balanced if each organ daily gained by 

 nutrition exactly as much as it lost daily by the discharge of 

 its function if excess of function were followed only by 

 such excess of nutrition as balanced the extra waste; it is 

 clear that there would occur no change in the relative sizes 

 of organs. But there is no such exact balance. If the excess 

 of function, and consequent excess of waste, is moderate, it is 

 not simply compensated by repair but more than compensated 

 there is a certain increase of bulk. This is true to some 

 degree of the organism as a whole, when the organism is 

 framed for activity. A considerable waste giving considerable 

 power of assimilation, is more favourable to accumulation of 

 tissue than is quiescence with its comparatively feeble assimi 

 lation: whence results a certain adaptation of the whole 

 organism to its requirements. But it is more especially true 

 of the parts of an organism in relation to one another. The 

 illustrations fall into several groups. The growth 



of muscles exercised to an unusual degree is a matter of com 

 mon observation. In the often-cited blacksmith s arm, the 

 dancer s legs and the jockey s crural adductors, we have 

 marked examples of a modifiability which almost every one 

 has to some extent experienced. It is needless to multiply 

 proofs. The occurrence of changes in the structure 



of the skin, where the skin is exposed to unusual stress of 

 function, is also familiar. That thickening of the epidermis 

 on a labourer s palm results from continual pressure and 



