INBRED AND HYBRID ANIMALS AND THEIR VALUE 331 



General Causes of Phenotypic Variation 



The contribution that genetics has made to an understanding of these 

 causes has been brought out clearly by Wright (18). With the aid of 

 diagrams he has emphasized the fact that, however complex the network 

 of processes involved in the development of a character may be, all processes 

 trace back to a gene action somewhere or an external stimulus somewhere. 



Wright's general treatment, particularly when applied to mammals, can 

 be extended by distinguishing two main paths by which genes can influence 

 a character. The more direct path is that tracing back from the character 

 to the genes in the individual bearing the character. A character, particu- 

 larly in mammals, may also be affected by the maternal environment, which, 



CHARACTER 



Fig. 132. 



in turn, is determined partly by the genetic constitution of the mother. 

 Therefore, a second path traces back through the maternal environment to 

 the genes of the mother. The maternal environment may also be afi'ected 

 by the grand-maternal environment, which, in turn, is determined partly 

 by the genes of the grandmother. The final result of this analysis is an 

 indefinite number of paths, which presumably have less importance the 

 further back they go (Fig. 132). 



The genes of the mother, grandmother, etc., are not the only genes 

 external to the individual that can influence a character. Nor is the 

 maternal environment the only medium through which such genes can act. 

 For example, the number of young raised to weaning is determined partly 

 by the genetic constitution of the young, and it has been known to aft'ect 

 not only such characters as gain in weight after birth, but also characters 

 influenced by the temperature of the nest. Variation in tangible environ- 

 mental factors of this nature can, of course, often be eliminated from an 

 experiment. To make our classification complete, however, the term 

 ''biological environment" will be used to group maternal environment with 

 all other environmental factors affected by the genetic constitution of the 



