26 THE EVOLUTION OF LIVING BEINGS. 



form, and if the reciprocal F t generations of 

 each cross are identical, there is reason to 

 suppose the tested form to be specifically 

 pure. 



2dly. submit as many of the F t individuals of each 

 cross to as extensive hybrid-analysis as possi- 

 ble, in order to test their constitutional identity. 

 If they prove to be of identical constitution in 

 all pointstested,thisfact,combined with theresult 

 obtained sub i, makes it very probable indeed, 

 that the tested individual is specifically pure. 

 It must be conceded that even this gives no absolute 

 certainty that the form tested is specifically pure in all 

 respects, but it is the best we can do. 



But even the simpler Davis-test has the great advan- 

 tage that we can say, that if two forms crossed recipro- 

 cally, give a heterogenous progeny, either one or the 

 other is, or both are, specifically impure, while, if the 

 progenies are each uniform, but not identical in re- 

 ciprocal crosses, there is no specific purity either. 



Because in these cases there is proof, that at least one 

 of the individuals crossed, forms more than one kind of 

 gametes, is polygametic and consequently a hybrid. 

 One class of differences within the species remains 

 to be considered : the non-transmittable effect of exter- 

 nal circumstances on the individuals composing the spe- 

 cies. That this can be considerable, shows f . i. a com- 

 parison of specimens of the same species growing at 

 high altitudes in the mountains, with specimens gro- 

 wing in the plains. These we will indicate as modi- 

 fications. 



