CHAPTER V. 



ADAPTATION. 



§ 67. In plants waste and repair being scarcely appre- 

 ciable, there are not likely to arise appreciable changes in the 

 proportions of already-formed parts. The only divergences 

 from the average structures of a species, which we may expect 

 particular conditions to produce, are those producible by the 

 action of these conditions on parts in course of formation; 

 and such divergences we do find. We know that a tree 

 which, standing alone in an exposed position, has a short 

 and thick stem, has a tall and slender stem when it grows 

 in a wood; and that also its branches then take a different 

 inclination. We know that potato-sprouts which, on reaching 

 the light, develop into foliage, will, in the absence of light, 

 grow to a length of several feet without foliage. And every 

 in-door plant furnishes proof that shoots and leaves, by 

 habitually turning themselves to the light, exhibit a certain 

 adaptation — an adaptation due, as we must suppose, to the 

 special effects of the special conditions on the still growing 

 parts. In animals, however, besides analogous struc- 



tural changes wrought during the period of growth, by sub- 

 jection to circumstances unlike the ordinary circumstances, 

 there are structural changes similarly wrought after maturity 

 has been reached. Organs that have arrived at their full 

 sizes possess a certain modifiability ; so that while the 



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