us distribution of a _species_, there being a gap of about a
thousand miles between its southern limits in Russia, and its reappearance
in the Alps. There are of course numerous instances in which species occur
in two or more islands, or in an island and continent, and are thus
rendered discontinuous by the sea, but these involve questions of changes
in sea and land which we shall have to consider further on. Other cases are
believed to exist of still wider separation of a species, as with the marsh
titmice and the reed buntings of Europe and Japan, where similar forms are
found in the extreme localities, while distinct varieties or sub-species,
inhabit the intervening districts.

_Extent and Limitations of Specific Areas._--Leaving for the present these
cases of want of continuity in a species, we find the most wide difference
between the extent of country occupied, varying in fact from a few square
miles to almost the entire land surface of the globe. Among the mammalia,
however, the same species seldom inhabits both the old and new worlds,
unless they are strictly arctic animals, as the reindeer, the elk, the
arctic fox, the glutton, the ermine, and some others. The common wolf of
Europe and Northern Asia is thought by many naturalists to be identical
with the variously coloured wolves of North America extending from the
Arctic Ocean to Mexico, in which case this will have perhaps the widest
range of any species of mammal. Little doubt exists as to the identity of
the brown bears and the beavers of Europe and North America; but all these
species range up to the arctic circle, and there is no example of a mammal
universally admitted to be identical yet confined to the temperate zones of
the two hemispheres. Among the undisputed species of mammalia the leopard
has an enormous range, extending all over Africa and South Asia to Borneo
and the east of China, and thus having probably the widest range of any
known mammal. The winged mammalia have not usually very wide ranges, there
being only one bat common to the Old and New Worlds. This is a British
species, _Vesperugo serotinus_, which is found over the larger part of
North America, Europe and Asia, as far {15} as Pekin, and even extends into
tropical Africa, thus rivalling the leopard and the wolf in the extent of
country it occupies.

Of very restricted ranges there are many examples, but some of these are
subject to doubts as to the distinctness of the species or as to