130 Heredity and Environment 



ably about 10 /U/A in diameter. Between these molecules and the 

 just visible particles of protoplasm there may be other units of 

 organization. These hypothetical particles of protoplasm have 

 been supposed by many authors to be the ultimate units of assimi- 

 lation, growth and division, and in so far as these units are sup- 

 posed to be the differential causes of hereditary characters, they 

 are known as inheritance units. 



Inheritance Units. It is assumed in practically all theories of 

 heredity that the "inheritance material," or the germinal proto- 

 plasm, is composed of ultra-microscopical inheritance units which 

 have the power of individual growth and division and which are 

 capable of undergoing many combinations and dissociations dur- 

 ing the course of development, by which combinations and 

 dissociations they are transformed into the structures of the 

 adult. Various names have been given to these units by 

 different authors; they are the "physiological units" of Herbert 

 Spencer, the "gemmules" of Darwin, the "plastidules" of Els- 

 berg and Haeckel, the "pangenes" of de Vries, the "plasomes" of 

 Wiesner, the "idioblasts" of Hertwig, the "biophores" and "de- 

 terminants" of Weismann. 



With the publication of Weismann's work on the germplasm 

 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 existence of inheritance units of some kind once 

 more became evident, and, without being able to define just what 

 such units are or just how they behave, modern students of hered- 

 ity assume their existence. They are now called determiners or 

 factors or genes, and they are usually thought of as units in the 

 germ cells which condition the characters of the developed or- 

 ganism, and which are in a measure independent of one another; 

 though of course neither they nor any other parts of a cell are 



