MIGRATION 175 



to do with finding a suitable place for breeding and for 

 bringing up the young. 



(b) In the life of many an animal the great difficulty of the 

 year is the winter, and many of the ways in which this 

 difficulty is met or evaded are familiar. Thus there may 

 be a winter-coma or a true hibernation (in mammals) ; 

 there may be a gathering of external stores ; there may be 

 an accumulation of internal reserves ; there may be a 

 protective change of colour ; but, in a way, the neatest 

 solution of all is migration. We may suppose, therefore, 

 that migration of birds arose in regions where a change of 

 climate made it difficult to face the winter. And even 

 without postulating much change of climate we may see 

 in the " partial migrations " which seem to be in process 

 of evolution in some cases, e.g. lapwings, to-day, the 

 advantage of seeking out optimum conditions — a cool 

 place with plenty of insects and water for the breeding 

 period, and a genial place, also with plenty of food, for the 

 winter's rest. 



(c) But there is a third factor to be recognised, the 

 advantage of dispersal from an area that tends to become 

 over-crowded after the breeding. 



Given a certain initiative, which animals rarely lack, 

 due either to the spur of necessity or the self-assertiveness 

 of an adventurous spirit, we can understand how birds 

 began the migrating device. It is one of the many answers- 

 back which they have given in the course of their struggle 

 for existence, and we have seen that its evolution would be 

 justified by at least three advantages. 



