152 MENDELISM chap, xin 



longiflora, 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. Ber- 

 nard and a toy terrier without recourse to artificial 

 fertilisation. Or sterility may be due to pathological 

 causes which prevent the gametes from meeting one an- 

 other in a healthy state. But in most cases it is prob- 

 able 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 im- 

 possible 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 mor- 

 phology or of systematics. It is a problem in physi- 

 ology. 



