NATURAL HISTORY OF SELBORNE 131 



kinds, prevents their crowding the one on the other. 

 Whether the swallows and house-martins return in the 

 same exact number annually is not easy to say, for reasons 

 given above ; but it is apparent, as I have remarked before 

 in my Monographies, that the numbers returning bear no 

 manner of proportion to the numbers retiring. 



