The Wallace-Darwin Correspondence 



has given me some facts showing that birds apparently 

 admire details of plumage. — Yours most sincerely, 



C. Darwin. 



Hurstpier point, if arc^ 24, [1868?]. 



Dear Darwin, — Many thanks for the photo, which I 

 shall get when I go to town. 



I return your son's notes with my notes on them. 



Without going into any details, is not this a strong 

 general argument ? — 



1. A species varies occasionally in two directions, but 

 owing to their free intercrossing they (the variations) never 

 increase. 



2. A change of conditions occurs which threatens the 

 existence of the species, but the two varieties are adapted 

 to the changing conditions, and, if accumulated, will form 

 two new species adapted to the new conditions. 



3. Free crossing, however, renders this impossible, and 

 so the species is in danger of extinction. 



4. If sterility could be induced, then the pure races 

 would increase more rapidly and replace the old species. 



5. It is admitted that partial sterility between varieties 

 does occasionally occur. It is admitted the degree of this 

 sterility varies. Is it not probable that Natural Selection 

 can accumulate these variations and thus save the species ? 



If Natural Selection can not do this, how do species 

 ever arise, except when a variety is isolated ? 



Closely allied species in distinct countries being sterile 

 is no difficulty, for either they diverged from a common 

 ancestor in contact, and Natural Selection increased the 

 sterility, or they were isolated, and have varied since, in 

 which case they have been for ages influenced by distinct 

 conditions which may well produce sterility. 



If the difficulty of grafting was as great as the difficulty 



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