200 Life and Death, Heredity and Evolution 



particular theoretical difficulty corresponds to an actual 

 one; that the direct effect of the environment on the body 

 cells is not inherited through the germ cells in the next 

 generation. It is only fair to say however that there is 

 certain evidence produced by the Austrian investigator 

 Kammerer (1913) in long-continued experimental studies on 

 amphibians which seems to imply such inheritance through 

 the germ cells of changes primarily produced in the body 

 cells of the animal. But practically all students of biology 

 will agree that this evidence is far from establishing heredity 

 of this sort, and that the overwhelming mass of evidence is 

 against it. 



But it is another question whether external agents may 

 not act directly on the germ cells, in such a way as to induce 

 them to produce a body with new characteristics, and to 

 transmit the same changes by cell division to the germ cells 

 that are to produce the later generations, so that these too 

 show the altered hereditary characters. This sort of action 

 would correspond to the hereditary changes produced by 

 external agents in the Protozoa and bacteria, such as we 

 described in Lecture 4. 



There is no theoretical difficulty whatever as to this ; the 

 difficulties are purely observational ones ; it turns out that 

 such changes do not occur so readily or frequently as one 

 would expect. There is no a priori reason why the sub- 

 stances of the germ cells should not be as readily altered 

 as any other mass of chemicals. But in practice it turns 

 out that most agents which produce chemical alterations 

 of the germ cells at the same time kill the organism. Further, 

 the germ cells, like other living systems, have a great 

 tendency to compensate for any disturbances induced them; 

 their condition is one of stable equilibrium, in which an alter- 

 ation is followed by a return to the original condition. Add 



