24 THE MIGRATION OF BIRDS. 



sedentary. Many more instances might be ad- 

 duced to show that Migration Habit is by no 

 means universal, even amongst the individuals of a 

 species ; and even amongst the migratory individuals 

 themselves the length of the journey varies to an 

 astonishing degree, some travelling double or more 

 the distance others travel. From the above facts, 

 we may also propound the law that wherever the 

 breeding area of a species intergrades with its 

 winter range, migration among individuals breed- 

 ing in the impinging districts has been suffered to 

 lapse. 



It is quite unnecessary here to deal with the 

 modification that migration has undergone in past 

 ages, having shown that it is not an unchanging 

 habit, that it is not a transmitted power from parent 

 to offspring in one immutable sequence ; we need 

 not therefore stay to inquire into its previous 

 history as regards individual species, or even 

 groups of the early ancestors of our existing avi- 

 fauna. No doubt species have acquired the habit, 

 and discarded it times without number in the course 

 of their eventful descent from remote ages, accord- 

 ing as conditions of life have favoured one or the 

 other end. Birds migrate from necessity, not from 

 choice ; in confirmation of which fact I may men- 

 tion that I do not know of any instance where some 

 or all of the individuals of a species quit their 

 breeding-grounds unless compelled to do so by 

 severity of climate, failure of food, or both. The 

 migrations of some species are shorter than others. 



