ROUTINE UNIFORM 261 



climatic changes and the relative abundance or 

 scarcity of enemies have influenced the course 

 of its evolution. These are all contributory 

 factors operating in the external environment. 

 But there are, besides, internal factors which 

 form part of the inherited constitution of the 

 bird, and, being passed on from generation to 

 generation, afford the conditions under which 

 migration is constantly being renewed. It is, I 

 believe, in this field of organic change and 

 relationship that the conditions of origin must 

 be sought. 



Just as the moth in passing from the rudi- 

 mentary to the perfect condition runs through a 

 series of changes, each one of which is marked 

 by a typical behaviour response adjusted to 

 meet some particular circumstance in the 

 external environment, so the annual history of a 

 bird displays an ordered routine, each phase of 

 which can be observed to correspond with one of 

 the successive changes in the environment. In 

 almost every direction, we find that this routine 

 is characterised, in broad outline, by great 

 uniformity ; so much so that, providing we 

 know the history of one species, we can forecast 

 with no small degree of certainty the general 

 course of behaviour of other members of the 

 family. But only the general course. There is 

 endless variation in just the particular way in 

 which the behaviour is adapted to meet the 

 needs of particular species — the major details 

 may be said to be specific, the minor details 

 varietal. 



