THE MECHANISM OF HEREDITY 67 



show that such hereditary differences behave as should 

 be expected if their inheritance follows the same laws 

 as do the simpler characters. The basis, as one might 

 say, of the Mendelian interpretation of size differences is 

 the proof that practically all qualitative characters are 

 affected by numerous factors. Sometimes there are two 

 or more factors which produce nearly identical visible 

 results, but more often the character complex is affected 

 in different ways and in various degrees by particular 

 factors. Whether the character develops at all or not 

 seems to be due to the presence or absence of one or more 

 main factors, but given the presence of these factors the 

 degree of development may be influenced by many sub- 

 sidiary factors or modifiers. Now these modifiers being 

 transmitted independently of one another and of the prin- 

 cipal factor or factors, an individual carrying certain 

 modifiers and lacking the principal factor may be crossed 

 with an individual carrying the main factor and lacking 

 the modifiers. The result is a series of recombinations 

 among the germ cells of the F^ generation which produces 

 F 2 individuals carrying various groups of modifiers and 

 therefore developing the character complex under con- 

 sideration in different degrees. 



If one studies carefully such crosses as the one just 

 described, he finds that a number of general conditions 

 are fulfilled. 



1. When pure or homozygous races are crossed, the 

 Fj populations are similar to the parental races in uni- 

 formity. This conclusion devolves from observations 

 that if any particular factors AA and aa are homozygous 

 in the parental races, they can only form Aa individuals 

 in the F^ generation. 



