44 THE ZOOLOGIST. 
serine birds, but are seldom recognised by those belonging to 
other families. 
The descent of an arctic climate upon the North Polar regions 
soon drove the residents of the Polar basin into more southern 
latitudes. Coast-birds like the Oystercatchers could escape by 
four routes. They could either follow the European or the 
American shores of the Atlantic, or the Asiatic or American 
shores of the Pacific. If they availed themselves of all four 
routes, the emigrants would soon become isolated in four colonies, 
which would eventually produce four species or groups of species. 
Behring’s Straits lies 25° south of the Pole, and the Asiatic coast 
is connected by a row of islands with the Pacific coast as far as 
40° from the Pole, so that isolation and consequently differentia- 
tion would begin late with the Pacific birds; whilst in the Atlantic 
the emigrants would probably be effectually isolated for ever by 
Greenland’s icy mountains at a distance of not more than 5° or 
7° from the Pole. It would therefore be reasonable to expect that 
the Pacific Oystercatchers would be much nearer allied to each 
other than the Atlantic Oystercatchers are. 
Let us now examine the peculiarities and geographical distri- 
bution of each species, and see how far our theories are supported 
by facts. 
1. Hematopus palliatus. — 'The North American Pied Oyster- 
catcher, like all the other Oystercatchers, has the entire head and 
neck nearly black, and the bill red. It resembles all the Pied 
Oystercatchers in having the greater wing-coverts, the upper tail- 
coverts, a considerable part of the secondaries, and the under 
parts below the neck, white. Further, it agrees with all the 
Oystercatchers of the New World in having pale flesh-coloured 
legs. Its specific character consists in having the rest of the 
plumage (i.e. the upper parts below the neck, with the 
exception of the greater wing-coverts and the upper tail-coverts) 
brown. 
Leaving Greenland to the left, the ancestors of the North 
American Pied Oystercatchers probably retreated from the Polar 
ice down Baffin’s Bay, and gradually extending their range south- 
wards along the Atlantic coast of North and South America, 
rounded Cape Horn, and turning northwards again along the 
Pacific coast of South America their descendants have extended 
their range northwards as far as Lower California, which appears- 
