ON VARIATIONS. 361 



hence in the next generation the development of the 

 brain (and probably the other nervous tissues) would 

 take place somewhat less vigorously, and the adult 

 brain be in consequence somewhat diminished in size. 

 In the next generation the diminution would be greater 

 still, and so on. 



Again, we have seen that in man, for instance, the 

 degree of (hereditary) pigmentation of the skin seems 

 to vary closely with the intensity of the heat and light 

 experienced. It is possible that the specific excretory 

 products of the pigment deposited in the skin, as a 

 direct response to the action of the environment, may 

 stimulate the pigment " determinants " in the germ- 

 plasm to increased vigour, so that in the next genera- 

 tion the organism will tend to become slightly more 

 pigmented than it had been in the previous one. Sup- 

 posing, on the other hand, the pigment cells of the skin 

 received no light rays whatsoever, as in animals which 

 had wandered into a subterranean cave, their metab- 

 olism would be reduced almost to nil, and so the pig- 

 ment " determinants " in the germ-plasm would 

 diminish in vigour, and the offspring of the animals 

 would be (at birth) somewhat less pigmented than they 

 had been in previous generations. 



It is obvious thajb on our specific secretion hypothesis 

 only a certain class of acquired characters can be in 

 any degree heritable; only those, in fact, of which the 

 corresponding tissues possess a specific secretion or ex- 

 cretion, capable of acting specifically on the " deter- 

 minants " of such tissues in the germ-plasm. For in- 

 stance, the blacksmith cannot transmit his brawny arm 

 in any degree to his descendants, as it is scarcely pos- 



