CHAPTER Y. 



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 structure 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 its branches then take a different inclin- 

 ation. 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 grow- 

 ing parts. In animals, however, besides analogous 

 structural changes wrought during the period of growth, 

 by subjection to circumstances unlike the ordinary circum- 

 stances ; there are structural changes similarly wrought, 

 after maturity has been reached. Organs that have 

 arrived at their full size, possess a certain modifiability ; 

 BO that while the organism as a whole, retains pretty 



