INHERITANCE OF ACQUIRED CHARACTERS 197 



gressive heredity, more or less taken for granted by writers of 

 the last century, was flatly denied by Dr. August Weismann, 

 who insisted that it was necessary that the theory of the 

 inheritance of characters acquired in the lifetime of the indi- 

 vidual should no longer be accepted without definite proof. 



In the theory of heredity through the development of the 

 germ cell controlled by influences exerted by structures within 

 the nucleus, Weismann found no room for the inheritance of 

 characters not preestablished within this germ. External in- 

 fluences in general cannot reach the germ cells, and throughout 

 nature the germ cells are elaborately protected from the direct 

 influence of external conditions. 



This attack upon an ancient theory roused its supporters 

 to defend their faith and to search for evidence to support it. 

 A temporary division of naturalists into two schools arose as a 

 result of this discussion. Those who held with Lamarck and 

 Spencer that characters gained in the life time of the individual, 

 and not received from ancestors possessing them, became hered- 

 itary, were known as Neo-Lamarckians. Those who, with 

 YVcismann, denied the existence of this factor and from a neces- 

 sity, real or fancied, laid special stress on the Darwinian principle 

 of natural selection, assumed the title of Neo-Darwinians. In 

 their hands the Darwinian principle became the all-powerful 

 factor in evolution, a theory of Allmacht which was soon 

 questioned from other quarters and by those not considered as 

 Neo-Lamarckians. Prominent among the leaders of the Neo- 

 Larnarckians were Herbert Spencer, Haeckel, Nageli, Cope, 

 Eimer, Hyatt, Gadow, Dall, Packard, and others. Among 

 the recognized Neo-Darwinians were Weismann, Wallace, Hux- 

 ley, Gray, Brooks, Lankester, and others. 



After some years of controversy, mostly theoretical, the dis- 

 cussion has been tacitly dropped by biologists generally. It is 

 recognized that the sole crucial test is that of experiment, that 

 experiment is not easy, inasmuch as it is very difficult to show 

 that any given trait in heredity really belongs to the category 

 of acquired characters, and that in no case has it been indu- 

 bitably shown that any character not inborn has been inherited. 

 Moreover the studies of the germ cell and the physical basis of 

 heredity tend to show that the structures of the germ cell are 

 more complex and that the processes of heredity are in a sense 

 more mechanical than could have been supposed in the time of 

 14 



