PHENOMENA OF INHERITANCE 279 



the other parent remain undeveloped. The 

 appearance of any developed character in an 

 organism depends upon many complicated 

 reactions of germinal units to one another and 

 to the environment. Under certain conditions 

 of the germ or of the environment some char- 

 acters may develop in hybrids to the exclusion 

 of their opposites whereas under other condi- 

 tions these results may be reversed or the char- 

 acters may be intermediate. The principle of 

 dominance is not a fundamental part of Men- 

 delian inheritance. Even when the characters 

 of hybrids are intermediate between those of 

 their parents, if the parental types reappear 

 in the F 2 generation we may be certain that 

 we are dealing with cases of Mendelian 

 inheritance. 



3. The Principle of Segregation. The in- 

 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 



