52 COLIN CLOUT'S CALENDAR. 



the field in the small portion of Africa and Asia fitted 

 for the evolution of such a race as the humming-birds, 

 and may thus effectually have prevented the eastern 

 swifts from ever developing in that direction. Of course 

 in any case the specialisation of humming-birds in 

 America must date back to a very remote period, both 

 on account of the profound modifications their form has 

 undergone, and on account of the immense number of 



O 



genera and species into which they have split up. 



