some parts of
tropical Asia, and six into arctic or temperate North America.

_Range of East Asian Birds._--To complete the evidence we only require to
know that the East Asiatic birds are as much like those of Europe, as we
have already shown to be the case when we take the point of departure from
our end of the continent. This does not follow necessarily, because it is
possible that a totally distinct North Asiatic fauna might there prevail;
and, although our birds go eastward to the remotest parts of Asia, their
birds might not come westward to Europe. The birds of Eastern Siberia have
been carefully studied by Russian naturalists and afford us the means of
making the required comparison. There are 151 species belonging to the
orders Passeres and Picariae (the perching and climbing birds), and of
these no less than 77, or more than half, are absolutely identical {39}
with European species; 63 are peculiar to North Asia, but all except five
or six of these are allied to European forms; the remaining 11 species are
migrants from South-eastern Asia. The resemblance is therefore equally
close whichever extremity of the Euro-Asiatic continent we take as our
starting point, and is equally remarkable in birds as in mammalia. We have
now only to determine the limits of this, our first zoological region,
which has been termed the "Palaearctic" by Mr. Sclater, meaning the
"northern old-world" region--a name now well known to naturalists.

_The Limits of the Palaearctic Region._--The boundaries of this region, as
nearly as they can be ascertained, are shown on our general map at the
beginning of this chapter, but it will be evident on consideration, that,
except in a few places, its limits can only be approximately defined. On
the north, east, and west it extends to the ocean, and includes a number of
islands whose peculiarities will be pointed out in a subsequent chapter; so
that the southern boundary alone remains, but as this runs across the
entire continent from the Atlantic to the Pacific ocean, often traversing
little-known regions, we may perhaps never be able to determine it
accurately, even if it admits of such determination. In drawing the
boundary line across Africa we meet with our first difficulty. The
Euro-Asiatic animals undoubtedly extend to the northern borders of the
Sahara, while those of tropical Africa come up to its southern margin, the
desert itself forming a kind of sandy ocean between them. Some of the