390 MIGRATIONS OF BIRDS. 



stance, which renders the periodical migration of Birds useful to 

 themselves or to their offspring, it is very evident that it is not 

 this, which is usually the cause that immediately produces the 

 change. Migrating Birds seem to experience, at certain periods 

 of the year, a necessity for changing their place ; as they expe- 

 rience at other times a desire of constructing nests, without being 

 led to it by any intellectual calculation, or by the anticipation of 

 the advantages which they will derive from it. It is a blind 

 instinct which guides them; and which sometimes developes 

 itself independently of everything, which would at the time 

 influence the well-being of the animal. Thus in experiments 

 made on some Birds of Passage, this need of change has shown 

 itself strongly at the usual time, though care has been taken to 

 maintain around them a constant temperature, and to give them 

 proper food; individuals having been selected for the experiment, 

 that had not yet formed the habit of migrating. When they 

 change their climate, they do not wait until the cold is become 

 insupportable to them, and are not gradually driven towards the 

 south by the approach of winter ; but they precede it, and go 

 almost at once into tropical climates. They frequently return in 

 spring, when the temperature is much below what it was at 

 their departure ; and in certain kinds the migrations do not corre- 

 spond with any distinguishable external circumstance. This 

 phenomenon is therefore inexplicable ; but in this it does not 

 differ from all those determined by instinct ; and as we advance 

 in the study of animals, we shall meet with many examples not 

 less interesting and incomprehensible. 



354. But because migrations depend on an instinctive and 

 blind impulse, we must not conclude that external circumstances 

 are without influence in developing in these Birds of Passage the 

 need they feel of change of place : it is remarked on the contrary, 

 that this phenomenon generally coincides with atmospheric varia- 

 tions ; and that the time of arrival or departure is often hastened 

 or delayed, according to the cessation or continuance of the 

 cold. 



355. The time at which Birds of Passage arrive in our country 

 or quit it, varies according to the species. Natives of the 

 northern countries of Europe come to us at the end of autumn 



