50 
ZOOLOGICAL GEOGRAPHY. 
[PART III. 
fernandensis, one of the Tyrannidte ; and two humming-birds, 
Eustephanus fernandensis and E. galeritus. 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 beyond this 
island lies another, called a Mas-a-fuero,” very much smaller; 
yet this, too, contains four species of similar birds ; one, 
Oxyurus mas-a-fuerm, allied to the wide-spread South Temperate 
0. spinicauda, and Cinclodes fusus, a South Temperate species — 
both Dendrocolaptidse ; with a humming-bird, Eustephanus ley - 
loldi , allied to the species in the larger island. The preceding 
facts are taken from papers by Mr. Sclater in the Ibis for 1871, 
and a later one in the same journal by Mr. Salvin (1875). The 
former author ha's some interesting remarks on the three species 
of humming-birds of the genus Eustephanus , above referred to. 
The Chilian species, E. galeritus, 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. leyboldi (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 ih Under the action of sexual selection (unchecked 
by some conditions which 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 the bird had established an isolated colony on 
Mas-a-fuera ; and here the process of change was continued in 
the male, but from 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 the phenomena can thus be accounted for by known laws, 
on the theory of very rare accidental immigrations from the 
