HUMMING-BIRDS. 147 



termed correlation of growth, or to the partial reap- 

 pearance of ancestral characters under more favourable 

 conditions, or to the direct action of changes of climate 

 and of food ; but they show us how varied and un- 

 accountable are the changes in specific forms that may 

 be effected in a comparatively short time, and by means 

 of very slight changes of locality. 



If now we consider the enormously varied conditions 

 presented by the whole continent of America the hot, 

 moist, and uniform forest-plains of the Amazon ; the 

 open llanos of the Orinoco ; the dry uplands of Brazil ; 

 the sheltered valleys and forest slopes of the Eastern 

 Andes ; the verdant plateaus, the barren paramos, the 

 countless volcanic cones with their peculiar Alpine 

 vegetation ; the contrasts of the East and West coasts ; 

 the isolation of the West Indian islands, and to a less 

 extent of Central America and Mexico which we know 

 have been several times separated from South America ; 

 and when we further consider that all these characteris- 

 tically distinct areas have been subject to cosmical and 

 local changes, to elevations and depressions, to diminu- 

 tion and increase of size, to greater extremes and greater 

 uniformity of temperature, to increase or decrease of 

 rainfall ; and that with these changes there have been 

 coincident changes of vegetation and of animal life, all 

 affecting in countless ways the growth and development, 

 the forms and colours, of these wonderful little birds 

 if we consider all these varied and complex influences, 

 we shall be less surprised at their strange forms, their 

 infinite variety, their wondrous beauty. For how many 

 ages the causes above enumerated may have acted upon 

 them we cannot say ; but their extreme isolation from 



L 2 



