214 BULLETIN NO. VII. 
terials under the guidance of natural selection, has solved 
every one. Let us watch the process, and, I doubt not, learn 
a lesson of skill and patience. The situation is a shallow pool 
which is destined to freeze nearly solid. It is grown up with 
rushes and Nuphar or spatter dock, two plants which play an 
important part in the domestic economy of this animal, supply- 
him at once with food and building materials. 
In the placid days of early autumn a pair of muskrats may be 
seen diving to the bottom and tugging and biting at the roots 
of the rush. After tearing them off, the rat collects four or 
five of the rushes, say four feet long, and swims with them to 
a spot selected over the deepest part of the pool. Here they 
are arranged in parallel order and carefully straightened. 
Then another mouthful is brought and placed across the first 
at right angles. The angles are bisected by other clusters 
until a circular raft 1s formed sufficiently strong to support the 
weight of the animal. Now the colony of rats sets at work in 
earnest. And all day long one or more rats may be seen on the 
platform apparently eating rushes. Closer inspection shows 
that they are biting rushes into short lengths to form the 
“filling” of the structure. The accumulation increases and 
its weight causes the raft to sink and a new series of long 
rushes is added. Thus repeatedly until sufficient material has 
been accumulated to rest on the bottom of the pond. All the 
previous weeks the rats seemed to make little progress, as the 
material sank as fast as lifted much above the surface. Dur- 
ing the early stages a strong wind may ruin the work of weeks, 
but the rats are never discouraged. 

“Porgy 
Fig. 12. Section of nee Rae hut. 
So far only vegetable matter has entered into the composi- 
tion of the hut, but as soon as it bégins to rise permanently 
above the water there isa change of method and the whole 
