286 HEREDITY AND ENVIRONMENT 



dividuality of inheritance units, and their seg- 

 regation or separation in the sex cells and 

 recombination in the zygote are fundamental 

 principles of the Mendelian doctrine. Indeed 

 the evidence for the individuality and contin- 

 uity of inheritance units is based entirely upon 

 such segregation and recombination, so that 

 the entire Mendelian theory may be said to 

 rest upon the principle of segregation. If 

 there are cases in which such segregation does 

 not take place they belong to other forms of 

 inheritance than the Mendelian ; if segregation 

 occurs in every instance there is no other type 

 of inheritance than that discovered by Mendel. 

 Are there cases which do not segregate ac- 

 cording to Mendelian expectation? 



When the Mendelian theory was new it was 

 generally supposed that there were forms of 

 inheritance which differed materially from the 

 Mendelian type; indeed it was supposed that 

 the latter was one of the less common forms 

 of heredity and that blending of parental 

 traits and not segregation was the rule. All 

 cases in which the characters of the parents 

 appeared to blend in the offspring or in which 



