species on either side penetrate and even cross the desert, but it is
impossible to balance these with any accuracy, and it has therefore been
thought best, as a mere matter of convenience, to consider the geographical
line of the tropic of Cancer to form the boundary. We are thus enabled to
define the Palaearctic region as including all north temperate Africa; and,
a similar intermingling of animal types occurring in Arabia, the same
boundary line is continued to the southern shore of the Persian Gulf.
Persia and Afghanistan undoubtedly belong to the Palaearctic region, and
Baluchistan should probably go with these. The boundary in the
north-western part of India is again difficult to determine, but it {40}
cannot be far one way or the other from the river Indus as far up as
Attock, opposite the mouth of the Cabool river. Here it will bend to the
south-east, passing a little south of Cashmeer, and along the southern
slopes of the Himalayas into East Thibet and China, at heights varying from
9,000 to 11,000 feet according to soil, aspect, and shelter. It may,
perhaps, be defined as extending to the upper belt of forests as far as
coniferous trees prevail; but the temperate and tropical faunas are here so
intermingled that to draw any exact parting line is impossible. The two
faunas are, however, very distinct. In and above the pine woods there are
abundance of warblers of northern genera, with wrens, numerous titmice, and
a great variety of buntings, grosbeaks, bullfinches and rosefinches, all
more or less nearly allied to the birds of Europe and Northern Asia; while
a little lower down we meet with a host of peculiar birds allied to those
of tropical Asia and the Malay Islands, but often of distinct genera. There
can be no doubt, therefore, of the existence here of a pretty sharp line of
demarkation between the temperate and tropical faunas, though this line
will be so irregular, owing to the complex system of valleys and ridges,
that in our present ignorance of much of the country it cannot be marked in
detail on any map.

Further east in China it is still more difficult to determine the limits of
the region, owing to the great intermixture of migrating birds; tropical
forms passing northwards in summer as far as the Amoor river, while the
northern forms visit every part of China in winter. From what we know,
however, of the distribution of some of the more typical northern and
southern species, we are able to fix the