436 THE PREFUNCTIONAL AS CONTRASTED WITH 



was found that it had become ten times as thick, that its cells 

 had developed striations very similar to those which characterise 

 heart-muscle, and that the whole bladder pulsated rhythmically.^ 



The other case is that of salamander larvae, brought up in water 

 which is deficient in oxygen. Such larvae show much enlarged 

 external gills, while the gills of specimens reared in water with 

 excess of oxygen are extremely small. In the enlarged gills, upon 

 which extra respiratory demands are being made, the capillaries 

 are larger, nearer to the surface, and the epithelium of the surface 

 and the endothelium of the capillaries are thinner, thus permitting 

 of a more rapid diffusion of gases. The converse changes are seen 

 in the reduced gills^ (fig. 212). 



§7 



It is important to note that no sharp line can be drawn between 

 functional responses of considerable morphogenetic extent, as in 

 the cases just cited, and transitory adjustments of a physiological 

 nature which leave no structural traces, such as a temporary local 

 vaso-dilation. The connexion between the degree of expansion of 

 melanophores and their rate of multiplication has been noted 

 above (p. 426). Further, it should be remembered that one and the 

 same kind of organ can respond by a morphogenetic change to one 

 degree of functional stimulus and not to another. For instance, it 

 appears that only severe demands on muscles will cause them to 

 hypertrophy; movements involving little mechanical strain, even 

 when rapid and prolonged, have no effect — e.g. those of knitting 

 or piano-playing. 



It must also be remembered that functional adaptation can only 

 take place within certain limits prescribed by heredity. The 

 thyroid responds very readily to the demands made upon it by in- 

 creasing or decreasing its supply of hormone and its size. Yet by 

 selection, it has been possible to establish separate genetic strains 

 in pigeons, a high-thyroid strain and a low-thyroid strain, which 

 differ from each other in the size and activity of their thyroids even 

 under identical external conditions^ (see also p. 409). 



Most important of all, it must be borne in mind that functional 

 modification may be very active in one group of animals, and 



1 Carey, 1921 b, 1924. ^ Drastich, 1925. ^ Riddle, 1929. 



