1864-1869] STERILITY OF HYBRIDS 295 



The cause of our difference, I think, is that I look at the Letter 213 

 number of offspring as an important element (all circum- 

 stances remaining the same) in keeping up the average 

 number of individuals within any area. I do not believe 

 that the amount of food by any means is the sole deter- 

 mining cause of number. Lessened fertility is equivalent 

 to a new source of destruction. I believe if in one district a 

 species produced from any cause fewer young, the deficiency 

 would be supplied from surrounding districts. This applies 

 to your Par. 5. 1 If the species produced fewer young from 

 any cause in every district, it would become extinct unless 

 its fertility were augmented through Natural Selection (see 

 H. Spencer). 



I demur to probability and almost to possibility of 

 Par. 1, as you start with two forms within the same area, 

 which are not mutually sterile, and which yet have sup- 

 planted the parent-form. 



(Par. 6.) I know of no ghost of a fact supporting 

 belief that disinclination to cross accompanies sterility. * It 

 cannot hold with plants, or the lower fixed aquatic animals. 

 I saw clearly what an immense aid this would be, but gave 

 it up. Disinclination to cross seems to have been independ- 

 ently acquired, probably by Natural Selection ; and I do 

 not see why it would not have sufficed to have prevented 

 incipient species from blending to have simply increased 

 sexual disinclination to cross. 



(Par. 1 1.) I demur to a certain extent to amount of 

 sterility and structural dissimilarity necessarily going to- 

 gether, except indirectly and by no means strictly. Look 

 at vars. of pigeons, fowls, and cabbages. 



I overlooked the advantage of the half-sterility of re- 

 ciprocal crosses ; yet, perhaps from novelty, I do not feel 

 inclined to admit probability of Natural Selection having 

 done its work so queerly. 



I will not discuss the second case of utter sterility, but 

 your assumptions in Par. 13 seem to me much too com- 

 plicated. I cannot believe so universal an attribute as 

 utter sterility between remote species was acquired in so 

 complex a manner. I do not agree with your rejoinder 

 on grafting : I fully admit that it is not so closely restricted 



1 See Letter 211. 



