urdus, the
thrush, and Hirundo, the swallow genus, are the only perching birds which
are truly cosmopolites; but there are many genera of hawks, owls, wading
and swimming birds, which have a world-wide range.

As a great many genera consist of single species there is no lack of cases
of great restriction, such as the curious lemur called the "potto," which
is found only at Sierra Leone, and forms the genus Perodicticus; the true
chinchillas found only in the Andes of Peru and Chili south of 9deg S. lat.
and between 8,000 and 12,000 feet elevation; several genera of finches each
confined to limited portions of the higher Himalayas, the blood-pheasants
(Ithaginis) found only above 10,000 feet from Nepal to East Thibet; the
bald-headed starling of the Philippine islands, the lyre-birds of East
Australia, and a host of others.

It is among the different genera of the same family that we meet with the
most striking examples of discontinuity, although these genera are often as
unmistakably allied as are the species of a genus; and it is these cases
that furnish the most interesting problems to the student of distribution.
{27} We must therefore consider them somewhat more fully.

Among mammalia the most remarkable of these divided families is that of the
camels, of which one genus Camelus, the true camels, comprising the camel
and dromedary, is confined to Asia, while the other Auchenia, comprising
the llamas and alpacas, is found only in the high Andes and in the plains
of temperate South America. Not only are these two genera separated by the
Atlantic and by the greater part of the land of two continents, but one is
confined to the Northern and the other to the Southern hemisphere. The next
case, though not so well known, is equally remarkable; it is that of the
Centetidae, a family of small insectivorous animals, which are wholly
confined to Madagascar and the large West Indian islands Cuba and Hayti,
the former containing five genera and the latter a single genus with a
species in each island. Here again we have the whole continent of Africa as
well as the Atlantic ocean separating allied genera. Two families (or
subfamilies) of rat-like animals, Octodontidae and Echimyidae, are also
divided by the Atlantic. Both are mainly South American, but the former has
two genera in North and East Africa, and the latter also two in South and
West Africa. Two other families of mammalia, though confined to the Eastern
hemisphere, are