ADAPTATION. 185 



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. We saw in the last chapter, that the actions of or- 

 ganisms entail re-actions on them ; and that specialities of 

 action entail specialities of re-action. Here it remains to be 

 pointed out, that the 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 sucn 

 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 each other. 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 multi- 

 ply proofs. The occurrence of changes in the struc- 

 ture of the skin, where the skin is exposed to a stress of 

 function, is also familiar. That thickening of the epidermis 

 on a labourer's palm, results from continual pressure and 

 friction, is certain : those who have not before exerted their 

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