HUMMING-BIRDS 329 



colony is formed, any stray bird which may come over adds 

 to the numbers, and checks permanent variation by cross- 

 breeding. 



We find, then, that all the chief peculiarities of the three 

 allied species of humming-birds which inhabit the Juan 

 Fernandez group of islands, may be fairly traced to the action 

 of those general laws which Mr. Darwin and others have 

 shown to determine the variation of animals and the per- 

 petuation of those variations. It is also instructive to note 

 that where the variations of colour and size have been greatest 

 they are accompanied by several lesser variations in other 

 characters. In the Juan Fernandez bird the bill has become 

 a little shorter, the tail feathers somewhat broader, and the 

 fiery cap on the head somewhat smaller ; all these peculiarities 

 being less developed or absent in the birds inhabiting Mas- 

 afuera. These coincident changes may be due, either to 

 what Mr. Darwin has termed correlation of growth, or to 

 the partial reappearance 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 plateaux, 

 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 characteristically 

 distinct areas have been subject to cosmical and local changes, 

 to elevations and depressions, to diminution and increase of 

 size, to greater extremes and greater uniformity of temper- 

 ature, 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 



