64 COLIN CLOUT'S CALENDAR. 



ttres themselves, which is actively exerted in the choice of 

 mates. Why some members of the swift and swallow 

 family should have undergone this change to humming- 

 birds in the western continent and not in the eastern would 

 be a more difficult question to answer offhand ; but I 

 fancy the difference may be partly due to two causes. 

 In the first place, the peculiar way in which the Old 

 "World is cut up into two distinct regions, hot and cold, 

 by the Mediterranean and the Himalayan range may 

 have favored extensive migration here ; while in Am- 

 erica the continuity of land, the warmth of summer, 

 and the general luxuriance of blossoms permit humming- 

 birds to range as far north as Canada ; and thus one con- 

 tinent may have favored only the old open insect-hunt- 

 ing types like the swift, while the other favored also 

 specialized flower-hunting types like the humming-birds. 

 In the second place, the creepers may already have occu- 

 pied 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 east- 

 ern swifts from ever developing in that direction. Of 

 course in any case the specialization 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 

 genera and species into which they have split up. 



