INTERNAL IMPULSE 279 



find numerous gradations, the lesser merging 

 step by step into the greater. Is the Swallow 

 a migrant and the Herring- Gull not ; is the 

 Tree-Pipit a migrant and the Bunting not ; 

 must a bird cross many miles of sea or of 

 land before it can be considered a migrant ; is 

 the length of the distance traversed a criterion of 

 migration ? Surely not. The distance traversed 

 is merely a collateral consequence of the process 

 as a whole. 



The annual life-history of a bird presents, as 

 we have seen, two distinct phases — the one in 

 which the individual dominates the situation, 

 the other in which it is subordinated to the 

 welfare of the community. Let us take these 

 two phases separately and endeavour to see how 

 they may have influenced the seasonal move- 

 ments ; and first let us take the more important 

 of the two, namely that one whicly is directly 

 concerned in the continuance of the race. 



In this phase we must consider the three 

 factors to which allusion has already been 

 made : — (1) the internal impulse, (2) the innate 

 ability to return to the former breeding ground, 

 (3) the conditions in the external environment. 

 These three work in close relation and, as I shall 

 endeavour to show, lead to important results. 



(1) If there were nothing in the inherited 

 nature beyond an impulse to seek the breeding 

 ground, if, that is to say, when the appropriate 

 locality were reached, the bird took no further 

 interest in the developing situation, the attain- 

 ment of reproduction would become largely a 



T 2 



