-such, _e.g._, as the family of mice and rats (_Muridae_), of
squirrels (_Sciuridae_), of guinea-pigs and spine-bearing porcupines
(_Hystricidae_), &c. The largest form of rodent is the capybara (or
river-hog of the Rio de la Plata),--which is preyed on by the jaguar.
Though a near ally of the little guinea-pig, it is as large as a hog.
Amongst the more interesting rodents may be mentioned beavers,[11] the
fur-bearing chinchilla, the jerboa (_Dipus_), the musk-rat (_Fiber_),
and the rat-mole (_Spalax_). The jerboa has very long hind legs, and a
habit of jumping, so that it resembles superficially (but not really) a
small kangaroo. The _Spalax_ is quite blind, and has the burrowing
habit, and somewhat the shape of the common mole. Some rodents are
fitted to flit through the air in long jumps, by means of the wide
extensibility of the skin of their flanks, which, when stretched out,
acts as a parachute. Such forms are the flying squirrels, and a curious
rodent called _Anomalurus_, from the exceptional clothing of the base of
its tail, which is furnished with large scales at its under part.

Another order of beasts may here be referred to, because it affords
interesting examples of the co-existence of external resemblance without
any real affinity. This order includes the insect-eating beasts, or
_Insectivora_, and comprises the moles, hedgehogs, shrew-mice (which are
not really "mice" at all), and their allies. The _Insectivora_ and
_Rodentia_ present us with a singular parallelism in the respective
modifications of structure, which are found in these two very distinct
orders. But the insectivorous forms (as might perhaps be expected from
their less abundant food) are always smaller in size than are the
parallel vegetable-eating groups of rodents. Indeed, one insectivore of
the genus _Sorex_ (the shrew-mouse genus) is the absolutely smallest
mammal which is known to exist.

As examples of the parallelism referred to may be mentioned the moles
(which resemble the rat-moles), the shrew-mice (which resemble true
mice), the hedgehogs, and the less known spiny tanrec of Madagascar
(which resemble porcupines in their clothing); certain graceful and
active tree-frequenting insectivores of the Indian Archipelago, _Tupaia_
(which resemble squirrels); an aquatic African form, _Potomogale_ (which
resembles the musk-rat); certain elephant shrews--long-legged, jumping,
African insectivores (which resemble the jerboa amongst rodents);