SUMMARY 209 



the nest, have been the main determining factors, 

 and have led to a wide divergence in its function. 

 At the one extreme the function is to insure 

 a plentiful supply of food for the young ; at the 

 other, to insure a station suitable for rearing 

 offspring. I took the Bunting and the Guillemot 

 as types of the two extremes. The young of 

 the former species are born in a very helpless 

 state. They are susceptible to cold and unable 

 to withstand prolonged exposure, and therefore 

 it is essential that there should be an ample 

 supply of food, upon which the parents can 

 draw liberally, in the vicinity of the nest. But 

 the nest is placed in a variety of situations, and 

 accommodation in this respect may be said to 

 be unlimited. The young of the latter species 

 are not so susceptible to exposure, and moreover 

 there is always an abundance of food in the 

 waters beneath the cliff; but ledges of rock, 

 upon which the egg can be securely deposited 

 and the young successfully reared, are limited 

 both in number and extent. The position then 

 is as follows : there are situations in plenty in 

 which hosts of Buntings can build their nests 

 but the supply of food is a difficulty, and if the 

 respective areas of different individuals were 

 insufficient to supply them with the necessary 

 food with the necessary rapidity, they would 

 run the risk of losing their offspring and the 

 species would not endure ; on the other hand, 

 cliffs upon which the Guillemot can rear' its 

 young are limited, but the supply of food 

 presents no difficulty, and consequently the 



