CHAPTER V. 

 ADAPTATION. 



67. IN plants, waste and repair being scarcely appro 

 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 



