TROPICAL NATURE 



half completed. In this small and comparatively barren 

 island (a mere rock, as it is described by some authors) there 

 would be no such constant abundance of food, and therefore 

 no possibility of a large permanent population of humming- 

 birds; while the climate would not differ materially from 

 that of the larger island. Variation would therefore be 

 checked, or might be stopped altogether; and we find the 

 facts exactly correspond to this view. The male, which had 

 already acquired his colour, remains almost undistinguishable 

 from his immediate ancestral form ; but he is a little smaller, 

 indicating either that the full size of that form had not been 

 acquired at the period of migration, or that a slight diminution 

 of size has since occurred, owing to a deficiency of food. The 

 female shows also a slight diminution of size, but in other 

 respects is almost exactly intermediate between the Chilian and 

 Juan Fernandez females. The colour beneath is light ashy, the 

 breast-spots are intermediate in size and colour, and the tail- 

 feathers have a large ill-defined white spot on the end of the 

 inner web which has only to be extended along the whole web 

 to produce the exact character which has been acquired in 

 Juan Fernandez. It seems probable, therefore, that the 

 female bird has remained nearly or quite stationary since its 

 migration, while its Juan Fernandez relative has gone on 

 steadily changing in the direction already begun; and the 

 more distant species geographically thus appears to be more 

 nearly related to its Chilian ancestor. 



Coming down to a more recent period, we find that 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 undergone 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. Keed 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 

 Fernandez, and thus keep up the stock; for it must be 

 remembered that whereas, at a first migration, both a male 

 and a female are necessary for colonisation, yet, after a 



