50 BOOK OF NATURAL HISTORY. 
the cracks with stones and mud, 
which they carry in their paws. 
It is in the summer that they 
do this. In autumn they lay 
up in their houses a store of 
bark, stripped from trees, for 
their food in winter. When 
cold weather comes, the mud 
freezes till it is almost as hard 
as stone ; but the beavers do 
not mind, for they are safe 
within. They have no need to 
go abroad for food, and no wild 
animal can reach them, for the 
the door of their houses is un- 
der water. They can swim 
and dive famously, and with 
their broad, scaly tails they can 
