HUMMING-BIRDS. 



145 



island, at a period when the male had acquired his per- 

 manent style of colouring, but was not quite so large as 

 he subsequently became ; while the change of the female 

 bird had not been half completed. In this small and 

 comparatively barren island (a mere rock, as it is de- 

 scribed by some authors) there would be no such constant 

 abundance of food, and therefore no possibility of a large 

 permanent population; while the climate would not 

 differ materially from that of the larger island. Varia- 

 tion 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 undistinguisliable from his imme- 

 diate ancestral form; but he is a little smaller, indicat- 

 ing 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 inter- 

 mediate 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 



I. 



