50 ZOOLOGICAL GEOGRAPHY. [part hi. 



fernandensis, one of the Tyrannid?e ; and two humming-birds, 

 Eustephanus fernandensis and E. gcderitus. The first is a wide- 

 spread South Temperate species, the two next are peculiar 

 to the island, while the last is a Chilian species which ranges 

 south to Tierra del Fuego. But ninety miles lieyond this 

 island lies another, called " Mas-a-fuero," very much smaller; 

 yet this, too, contains four species of similar birds ; one, 

 Oxyurus mas-a-fuera;, allied to the wide-spread South Temperate 

 0. spinicauda, and Cinclodes Jusus, a South Temperate species — 

 both Dendrocolaptidse ; with a humming-bird, Eustephamis hy- 

 boldi, allied to the species in the larger island. The preceding 

 facts are taken from papers by Mr. Sclater in the This foi- 1871, 

 and a later one in the same journal by Mr. Salvin (1875). The 

 former author has some interesting remarks on the tliree species 

 of humming-birds of the genus Eustephanus, above referred to. 

 The Chilian species, E. gakritus, is green in both sexes. E. 

 fernandensis has the male of a fine red colour and the female 

 green, though differently marked from the female of E. galeritus. 

 E. leyholdi (of Mas-a-fuera) has the male also red and the female 

 green, but the female is more like that of E. galeritus, than it is 

 like the female of its nearer ally in Juan Fernandez. Mr. 

 Sclater supposes, that the ancient parent form of these three 

 birds had the sexes alike, as in the present Chilian bird ; that a 

 pair (or a female having fertilised ova) reached Juan Fernandez 

 and colonised it. Under the action of sexual selection (unchecked 

 by some conditions Mdiich had impaired its efficacy on the con- 

 tinent) the male gradually assumed a brilliant plumage, and the 

 female also slightly changed its markings. Before this change 

 was completed tlie bird had established an isolated colony on 

 Mas-a-fuera ; and here the process of change was continued in 

 the male, but froni some unknown cause checked in the 

 female, which thus remains nearer the parent form. Lastly 

 the slightly modified Chilian bird again reached Juan Fer- 

 nandez and exists there side by side with its strangely altered 

 cousin. 



All th(> ]ihenomena can thus be accounted for Ijy known laws, 

 on the theory of very rare accidental immigrations from the 



