98 Darwin, and after Darwin. 



during the process of specific differentiation, instead 

 of after that process had been fully completed ? 



2. Can you doubt, after duly considering the cir- 

 cumstances under which allied species of plants have 

 been differentiated viz. in ninety-five per cent, of 

 cases intimately commingled on common areas, and 

 therefore under identical environments that cross- 

 infertility must have arisen before or during the 

 specific differentiation? 



3. Can you doubt, after duly considering the facts 

 of prepotency on the one hand and those of Jordan's 

 physiological varieties on the other, that cross-infer- 

 tility does arise before or during the specific differen- 

 tiation ? 



4. If you cannot express a doubt upon any of these 

 points, can you explain why you refuse to accept the 

 theory of the origin of species by means of physio- 

 logical selection, together with the explanation which 

 this theory affords of the continued cross-fertility of 

 domesticated varieties ? 



5. Supposing this theory to be true, can you con- 

 ceive of any other classes of facts which, either 

 quantitatively or qualitatively, could more directly or 

 more effectually prove its truth than those which have 

 now been adduced ? 



On these five heads I entertain no doubt. I am 

 convinced that the theoiy of physiological selection is 

 the only one that can explain the facts of inter-specific 

 sterility on the one hand, and, on the other hand, the 

 contrast which these facts display to the unimpaired 

 fertility of our domesticated varieties. 



In conclusion, it seems d< sirable once more to insist 

 that there is no antagonism or rivalry between the 



