100 HEREDITY AND ENVIRONMENT 



heredity that the "inheritance material," or 

 more correctly the germinal protoplasm, is 

 composed of ultra-microscopical units which 

 have the power of individual growth and di- 

 vision and which are capable of undergoing 

 many combinations and dissociations during 

 the course of development, by which combina- 

 tions and dissociations they are transformed 

 into the structures of the adult. Various 

 names have been given to such units by differ- 

 ent authors ; they are the "physiological units" 

 of Herbert Spencer, the "gemmules" of Dar- 

 win, the "plastidules" of Elsberg and Haeckel, 

 the "pangenes" of de Vries, the "plasomes" of 

 Wiesner, the "idioblasts" of Hertwig, the "bio- 

 phores" and "determinants" of Weismann. 



With the publication of Weismann's work 

 on the germ-plasm in 1892 speculation with 

 regard to these ultra-microscopic units of life 

 and of heredity reached a climax and began 

 to decline, owing to the highly speculative 

 character of the evidence as to the existence, 

 nature and activities of such units. But with 

 the rediscovery of Mendel's principles of 

 heredity the necessity of assuming the exist- 



