118 MAMMALS

him to the place where it is to be sunk.’ Many of the
beaver dams are three or four hundred feet long, and
perhaps a dozen feet or more in width, with occasion-
ally a single furrow in the crest for the water to escape
through when the stream is at the average level.
Some of these constructions are 1,000 years old, and
owing to the work of the beaver in the present and
the past large tracts of country have been covered
with water, and meadows formed where once the
forest grew.

The beavers are the last of the Sccuromorpha ; in
the Myomorpha there are four families—Myoxde,
consisting of the dormice; Murid@, comprising the
other mice and rats and voles ; Geomyzde, represented
by the American pouched rat; and Dizpodid@, embrac-
ing the jerboas. In all these, as in the hares and
rabbits, the tibia and fibula are joined at the end,
whereas among the squirrels the shank-bones are
always separate. The dormice are recognisable at
once by their bushy tails, the jerboas by their long
hindlegs. The mouse family are represented all over
the world, even in Australia; and it is no exaggera-
tion to say that they live in millions. In Germany
the hamsters are so numerous that in one year in
one district alone 100,000 were killed; in America
the white-footed mouse is as common as the ordinary
mouse is in England; the voles occasionally swarm
in such numbers as to constitute a veritable plague ;
the lemmings migrate in huge armies across Norway,
clearing everything in their track, like so many ants
keeping straight on, even swimming rivers and climb-
ing mountains, crowding on in countless thousands