range of the Cetacea._--It is natural to suppose that the
geographical range of the different species of Cetacea should be less
correctly ascertained than that of the terrestrial mammifers. It is,
however, well known that the whales which are obtained by our fishers in
the South Seas are distinct from those of the North; and the same
dissimilarity has been found in all the other marine animals, of the
same class, so far as they have yet been studied by naturalists.

_Dispersion of quadrupeds._--Let us now inquire what facilities the
various land quadrupeds enjoy of spreading themselves over the surface
of the earth. In the first place, as their numbers multiply, all of
them, whether they feed on plants, or prey on other animals, are
disposed to scatter themselves gradually over as wide an area as is
accessible to them. But before they have extended their migrations over
a large space, they are usually arrested either by the sea, or a zone of
uncongenial climate, or some lofty and unbroken chain of mountains, or a
tract already occupied by a hostile and more powerful species.

_Their powers of swimming._--Rivers and narrow friths can seldom
interfere with their progress; for the greater part of them swim well,
and few are without this power when urged by danger and pressing want.
Thus, amongst beasts of prey, the tiger is seen swimming about among
the islands and creeks in the delta of the Ganges, and the jaguar
traverses with ease the largest streams in South America.[878] The bear,
also, and the bison, cross the current of the Mississippi. The popular
error, that the common swine cannot escape by swimming when thrown into
the water, has been contradicted by several curious and
well-authenticated instances during the floods in Scotland of 1829. One
pig, only six months old, after having been carried down from Garmouth
to the bar at the mouth of the Spey, a distance of a quarter of a mile,
swam four miles eastward to Port Gordon, and landed safe. Three others,
of the same age and litter, swam, at the same time, five miles to the
west, and landed at Blackhill.[879]

In an adult and wild state, these animals would doubtless have been more
strong and active, and might, when hard pressed, have performed a much
longer voyage. Hence islands remote from the continent may obtain
inhabitants by casualties which, like the late storms in Morayshire, may
only occur once in many centuries, or thousands of years, under all the
same