56 DISPOSITION TO SECURE A TERRITORY 



histories of other species gave no further support 

 to our interpretation, if, in short, the evidence 

 were to break down at this point, then we 

 should be forced to seek some other explana- 

 tion more in keeping with the general body of 

 facts. 



But far from placing any obstacle in the way 

 of an interpretation in terms of inherited disposi- 

 tion, the behaviour of many of those residents 

 which are not paired for life gives us even surer 

 ground for that belief Moreover in their case 

 the initial stages in the process are more acces- 

 sible to observation. I will endeavour to explain 

 why. In the process of reproduction the 

 environment has its part to play — whether in the 

 manner here suggested, or indirectly through 

 the question of food-supply, matters not at the 

 moment. Now, migratory species are more 

 highly specialised than resident species as 

 regards food, and are affected more by variations 

 of temperature, so that they can live for only a 

 part of the year in the countries which they visit 

 for the purpose of procreation. Hence the 

 organic changes, which set the whole process in 

 motion, must be coincident in time with the 

 growth of appropriate conditions in the environ- 

 ment ; for if it were not so, if the internal 

 organic changes were to develop prematurely, 

 the bird would 'undertake its journey only to 

 find an insufficiency of food upon its arrival, and 

 this would scarcely contribute towards survival. 

 Definite limitations have therefore been imposed 

 upon the period of organic change. But in the 



