INDEPENDENT MENDELIAN INHERITANCE 103 



in a certain experiment. There is always a chance that biological 

 conditions of necessity may disturb a ratio, for after all a ratio is only 

 the end point of a series of phenomena which we pretend to describe 

 step by step. Unless constantly guarded against, such biological con- 

 ditions as differences in viability, variations in phenotypic expression, 

 etc., may result in selective elimination of a certain number of zygotes 

 at some time previous to classification, or in error in the classification 

 of some individuals. 



