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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