174 Changing Plant and Animal ^Nature 



less glands, manifested striking peculiarities in their develop- 

 ment (Fig. 46). These results have been verified and 

 extended and show very definitely that the character of the 

 development is related to the characters of the metabolism, 

 that this in turn is not merely a question of what is in- 

 herited but also a question of material income and of 

 chemical actions and reactions. In the examples given, the 

 individual is taken for granted, the modifications operat- 

 ing at some point after the fertilized q%% has started its 

 development. 



There is unmistakable evidence that the environment 

 does in many ways bring about variations. Indeed there are 

 many people who question the value of any effort to improve 

 conditions of living, whether for human beings or for cattle, 

 unless there is some hope that the recognized effects of such 

 improvement will in some way carry over to subsequent 

 generations and render eventually further effort unnecessary. 

 This problem is closely tied up with the whole question of 

 what is and what is not inherited. While the tendency among 

 the professional biologists is to question the heritability of 

 any modification, a large part of the difficulty lies in our 

 terminology, or rather in recognizing clearly what is a modi- 

 fication and what is a normal manifestation of inherent 

 capacity. 



TJse and Disuse 



From the point of view of evolution it is important to 

 ask further whether any of the modifications brought about 

 either by the direct action of the environment upon the 

 organism, or by the response of the organism to the environ- 

 ment, are ever transmitted to the offspring. We are all fa- 

 miliar with the effects of use and disuse. Exercise leads to 

 growth; the blacksmith's arms are stronger than the priest's. 

 Primitive people who eat harsh food have in general better 

 teeth and larger jaws than most of our own neighbors. Prac- 

 tice increases skill. We know also that on the other hand 



