24 THE EVOLUTION OF LIVING BEINGS. 



istly. In cases where one and the same individual 

 forms one kind of male gametes and one, but a 

 different kind of female gametes, the crossed 

 offspring of two such individuals would in F! 

 be uniform also, because each child is the pro- 

 duct of the combination of a female gamete 

 with a male one and all female gametes as well 

 as all male gametes are alike, allthough the male 

 ones differ from the female ones. 



Such a state of affairs can however easily be 

 detected by the fact that the reciprocal crosses 

 will be different, thus indicating that there 

 must be something amiss with the specific puri- 

 ty of the tested form. We must therefore make 

 the Davis-test more severe by extending it as 

 follows : 



Specific purity is indicated by the uniformity 

 and identity of the F^ generations obtained by 

 crossing the individuals to be tested, RECIPROCALLY. 

 2dly But in the following case even this more severe 

 test would remain indecisive viz, if there were 

 preferential mating between the gametes of two 

 hybrids, crossed together in such a way, that only 

 one kind of the different gametes produced by 

 the one hybrid were able to combine with one 

 kind,only of the different gametes produced by 

 the other. In such a case the F! generations, 

 even the reciprocal ones, would be uniform and 

 identical and thus simulate specific purity of 

 the tested individuals. 



In dealing with plants however, such cases 



