358 ACTION OF NATURAL SELECTION 



vironment simultaneously with the body tissues. As 

 Weismann points out, a necessary corollary to this view 

 is " the assumption of material determinants which 

 exist in the germ-plasm and are passed on from one 

 generation to another." If change of environment 

 acts cumulatively on the fleece of the sheep, or the 

 structural characters of a dog, it follows that it must 

 in each of the first few generations act also on the " de- 

 terminants " in the germ-plasm specifically represent- 

 ing such specific characters. The effect produced on 

 such determinants in the first generation must serve 

 more or less as a starting point for the environment to 

 work upon still further in the next generation, and 

 so on. 



Through what agency is the environment enabled to 

 act on the germ-plasm? To me the only conceivable 

 one is a chemical influence, through products of metab- 

 olism and specific internal secretions. We have seen 

 in a previous chapter that the products of metabolism 

 of an organism may exert a retarding effect on its 

 own growth, and in some cases a stimulating effect on 

 the growth of other organisms. Physiological re- 

 search of the last few years has shown that most 

 of the organs and tissues of the body have specific 

 internal secretions, which, passing into the general cir- 

 culation, may exert an influence of vital importance on 

 the general metabolism of the organism. Thus extir- 

 pation of the thyroid gland produces symptoms which 

 in many animals end fatally, but which may be dimin- 

 ished or suppressed by feeding on the gland substance, 

 or injection of extracts of it. Extirpation of the 

 suprarenal glands results in much more speedy death, 





