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. 
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