238 Heredity and Environment 



Weismann's Theories. Weismann introduced a new era in 

 biology by denying the inheritance of all kinds of acquired char- 

 acters, and by challenging the world to produce evidence that 

 would stand a rigorous analysis. But Weismann's greatest ser- 

 vice lay in his constructive theories rather than in destructive 

 criticism; he forever disposed of theories of pangenesis and the 

 like by showing that the germ cells are not built up by contribu- 

 tions from the body and that characters are not transmitted from 

 generation to generation; but on the other hand that there is 

 transmitted a germplasm which is relatively independent of the 

 body and which te relatively very stable in organization. This 

 epoch-making theory of Weismann's has naturally undergone 

 some changes, as the result of new discoveries. It is no longer 

 believed that the germplasm is really independent of the body, 

 nor that it is absolutely stable, as Weismann at one time held. 

 There is no doubt that the germ cells and the germplasm are 

 physiologically related to other cells and to other plasms, and 

 similarly there is no doubt that the germplasm although very 

 stable can and does change its constitution under some rare 

 conditions. But in the main the germplasm theory is accepted by 

 the great majority of biologists to-day, and recent work in 

 genetics and cytology has brought many confirmations of this 

 theory. 



Distinction between Hereditary and Acquired Characters. As 

 long as it was believed that the developed characters of an or- 

 ganism could be transmitted as such to its descendants it was cus- 

 tomary to speak of developed characters as hereditary or ac- 

 quired and to talk of the inheritance or non-inheritance of acquired 

 characters. This distinction is not a logical one for all developed 

 characters are invariably the result of the responses of the ger- 

 minal organization to environmental stimuli; and of course no 

 developed character can be purely hereditary or purely environ- 

 mental. But when a given character arises in many individuals 

 of the same biotype under different environmental conditions it 



