292 RELATION OF TERRITORY TO MIGRATION 



food will be impossible, or at any rate difficult 

 to obtain at certain seasons. Hence there will 

 come a time when the area of association ceases 

 to follow in the wake of the expansion, and 

 the breeding area begins to diverge from the 

 subsistence area. 



How, then, is the gulf between these two 

 areas to be bridged ? We can of course say 

 that those individuals which, in virtue of some 

 slight variation of hereditary tendency, return 

 to regions where food is plentiful will survive ; 

 whilst others, less well endowed, will perish. 

 We can state the position in some such general 

 terms, and doubtless there would be truth in 

 the statement, but it does not carry us far ; 

 we wish to know more of the nature of the 

 tendency, and of the manner in which it has 

 evolved. Well now, in this new situation 

 which arises, two things are apparent that 

 the struggle for existence becomes a struggle 

 for the means of subsistence, and that anything 

 in the inherited constitution of the bird which 

 can be organised to subserve the biological end 

 in view becomes of selection value. So long as 

 food can always be procured in the new areas of 

 association, the individuals that behave in 

 accordance with ancestral routine gain thereby 

 no particular advantage ; but directly the 

 breeding range extends into regions where 

 the supply fluctuates, traditional experience 

 becomes a factor in survival, and those indi- 

 viduals that come under its influence will, on 

 the average, be more likely to endure and so 



