146 TEOPICAL NATUIIE, AND OTHER ESSAYS. 



the comparatively small and dull-coloured Chilian bird 

 has again migrated to Juan Fernandez ; but it at once 

 came into competition with its red descendant, which 

 had firm possession of the soil, and had probably under- 

 gone slight constitutional changes exactly fitting it to 

 its insular abode. The new-comer, accordingly, only just 

 manages to maintain its footing ; for we are told by 

 Mr. Ueed, of Santiago, that it is by no means common ; 

 whereas, as we have seen, the red species is excessively 

 abundant. We may further suspect that the Chilian 

 birds now pass over pretty frequently to Juan Fernan- 

 dez, and thus keep up the stock ; for it must be remem- 

 bered that whereas, at a first migration, both a male and 

 a female are necessary for colonization, yet, after a 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 varia- 

 tions of animals and the perpetuation of those varia- 

 tions. 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 



