Alfred Russel Wallace 



from any cause in every district, it would become extinct 

 unless its fertility were augmented through Natural Selec- 

 tion {see H. Spencer). 



I demur to the probability and almost to the 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 accom- 

 panies 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 independently 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. 11 : I demur to a certain extent to amount of 

 sterility and structural dissimilarity necessarily going 

 together, except indirectly and by no means strictly. 

 Look at the case 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 the probability of Natural Selection 

 having done its work so clearly. 



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

 your assumptions in par. 13 seem to me much too compli- 

 cated. I cannot believe so universal an attribute as utter 

 sterility between remote species was acquired in so com- 

 plex a manner. I do not agree with your rejoinder on 

 grafting; I fully admit that it is not so closely restricted 

 as crossing; but this does not seem to me to weaken the 

 case as one of analogy. The incapacity of grafting is like- 

 (\dse an invariable attribute of plants sufficiently remote from 

 each other, and s^ome times of plants pretty closely allied. 



The difficulty of increasing the sterility, through Natural 



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