120 MAMMALS

to be drowned in the sea. The black rat came from
Asia in the twelfth century to swarm allover Europe ;
the brown rat, from Central Asia, swam the Volga in
thousands in 1727, to almost wipe out the black rat,
and spread not only over Europe, but take ship to
America and there increase quite as alarmingly. So
plentiful are the brown rats of Paris that over 16,000
a month are killed in the slaughterhouses. Some of
the small rodents are really beautiful, one of the
prettiest being the harvest mouse, the smallest but one
of our native mammals, which was discovered by Gil-
bert White of Selborne, and makes a nest of woven
grass blades about as big as a cricket ball, and feeds
its young from the outside of it, clinging to the grass
as it does so by its prehensile tail.

The third group of rodents with only two incisor
teeth is that containing the porcupines. Most of these
are American. The coypu, known in commerce and in
its South American home as the nutria, is one of the
largest of the family, and is often over two feet long ;
the West Indian hutias also attain respectable sizes.
Another conspicuous member of this group is the
porcupine—that is, the ‘spiny pig,’ called a pig be-
cause of its grunting powers—which is found in Europe,
Asia, and Africa, and has allies in America which
climb trees and have prehensile tails. The largest of
all the rodents is the South American capybara, whose
habits are those of a hippopotamus and who is nearly a
yard in length. Close akin to him in structure are the
cavies or guinea-pigs, so-called owing to a limited
knowledge of geography which confused Guinea on the
West Coast of Africa, where the cavies do not exist,