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 NupJiar 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 is 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. 



Fig. 12. Section of muskrat hut. 



So far only vegetable matter has entered into the composi- 

 tion of the hut, but as soon as it begins to rise permanently 

 above the water there is a change of method and the whole 



