NATURAL SELECTION AS APPLIED TO BIRDS 321 



prey no less viciously upon the eggs and young of their own 

 species. And this inter-necine warfare becomes intensified 

 through the I extraordinary pugnacity of their own nestlings. 

 Never exceeding two in number in each nest, these chicks, 

 while yet in their downy plumage, will " fight tooth and nail 

 with one another over some trivial bit of food, locked each to 

 the other by every claw, and fighting with loud squeals as they 

 used their tiny beaks." " It is a noticeable fact," continues 

 Dr. Wilson, " in connection with this bird that only one of the 

 two hatched in a nest survives. This is connected with the 

 tendency of the young to wander and get separated, and also 

 with their tendency to fight, and with the instinct which teaches 

 the parent to be chary of giving them too much nursing. The 

 consequence of all this is that while the mother is engrossed 

 with one, the other wanders out of reach and is sooner or later 

 snapped up by a hungry neighbour." 



Selection here certainly secures the survival of the strongest 

 and most pugnacious individuals of each generation ; and tends 

 at the same time to preserve and increase raptorial structural 

 characters — large claws and beak. And the Skuas are re- 

 markable for these particular characters. 



The subject of intra-specific selection among birds was very 

 lightly touched upon by Darwin. He selected but two in- 

 stances, drawing numerous others from other groups of the 

 animal kingdom. 



Under the heading "The Struggle for Life most Severe 

 between Individuals and Varieties of the Same Species," he 

 wrote : — 



" As the species of the same genus usually have, though by 

 no means invariably, much similarity in habits and constitution, 

 and always in structure, the struggle will generally be more 

 severe between them, if they come into competition with each 

 other, than between the species of distinct genera. We see 

 this in the recent extension over parts of the United States of 

 one species of Swallow having caused the decrease of another 

 species. The recent increase of the Missel-thrush in parts of 

 Scotland has caused the decrease of the Song-thrush." 



The species of Swallows were not mentioned by Darwin, 

 and though American writers have several times quoted this 

 passage for their several purposes, and always with tacit ap- 



