, being found only on the banks of the Volga and Don in
South-eastern Russia, while the other, _M. pyrenaica_, is confined to
streams on the northern side of the Pyrenees. In tropical America there are
four different kinds of bell-birds belonging to the genus Chasmorhynchus,
each of which appears to inhabit a restricted area completely separated
from the others. The most northerly is _C. tricarunculatus_ of Costa Rica
and Veragua, a brown bird with a white head and three long caruncles
growing upwards at the base of the beak. Next comes _C. variegatus_, in
Venezuela, a white bird with a brown head and numerous caruncles on the
throat, perhaps conterminous with the last; in Guiana, extending to near
the mouth of the Rio Negro, we have _C. niveus_, the bell-bird described by
Waterton, which is pure white, with a single long fleshy caruncle at the
base of the beak; the last species, _C. nudicollis_, inhabits South-east
Brazil, and is also white, but with black stripes over the eyes, and with a
naked throat. These birds are about the size of thrushes, and are all
remarkable for their loud, ringing notes, like a bell or a blow on an
anvil, as well as for their peculiar colours. They are therefore known to
the native Indians wherever they exist, and we may be the more sure that
they do not spread over the intervening areas where they have never been
found, and where the natives know nothing of them.

A good example of isolated species of a group nearer home, is afforded by
the snow-partridges of the genus Tetraogallus. One species inhabits the
Caucasus range and nowhere else, keeping to the higher slopes from 6,000 to
11,000 feet above the sea, and accompanying the ibex in its wanderings, as
both feed on the same plants. Another {25} has a wider range in Asia Minor
and Persia, from the Taurus mountains to the South-east corner of the
Caspian Sea; a third species inhabits the Western Himalayas, between the
forests and perpetual snow, extending eastwards to Nepal; while a fourth is
found on the north side of the mountains in Thibet, and the ranges of these
two perhaps overlap; the last species inhabit the Altai mountains, and like
the two first appears to be completely separated from all its allies.

There are some few still more extraordinary cases in which the species of
one genus are separated in remote continents or islands. The most striking
of these is that of the tapirs, forming the genus Tapirus, of which there
are two o