xiii VARIATION AND EVOLUTION 145 



way as soon as we are in a position to apply the 

 test of breeding. Mendelism has helped us to 

 realise that specific characters may be but incidental 

 to a species — that the true criterion of what con- 

 stitutes a species is sterility, and that particular form 

 of sterility which prevents two healthy gametes on 

 uniting from producing a zygote with normal powers 

 of growth and reproduction. For there are forms of 

 sterility which are purely mechanical. The pollen 

 of Mirabilis jalapa cannot fertilise M. longifiora, 

 because the pollen tubes of the former are not long 

 enough to penetrate down to the ovules of the latter. 

 Hybrids can nevertheless be obtained from the 

 reciprocal cross. Nor should we expect offspring 

 from a St. Bernard and a toy terrier without recourse 

 to artificial fertilisation. Or sterility may be due to 

 pathological causes which prevent the gametes from 

 meeting one another in a healthy state. But in 

 most cases it is probable that the sterility is due to 

 some other cause. It is not inconceivable that 

 definite differences in chemical composition render 

 the protoplasm of one species toxic to the gametes 

 of the other, and if this is so it is not impossible 

 that we may some day be able to express these 

 differences in terms of Mendelian factors. The very 

 nature of the case makes it one of extreme difficulty 

 for experimental investigation. At any rate, we 

 realise more clearly than before that the problem of 

 species is not one that can be resolved by the study 

 of morphology or of systematics. It is a problem 

 in physiology. 



