Impulse to Build. 99 



5. When we find different methods of building 

 practised by the same animal, we generally observe 

 the same uniformity in carrying out each of these 

 methods, as we find among animals having only 

 one method. 



The house of the Muskrat, built of mud and 

 reeds in shallow waters, and the burrow of the 

 same animal where he can find steep banks, are two 

 methods by which he adapts himself to the differ- 

 ent conditions of the places he inhabits ; but each 

 method is as uniform, in itself considered, as though 

 that were the only method practised by the ani- 

 mal. 



In a certain sense, the structure of an animal's 

 organs and the functions of his body have a relation 

 to the home he prepares, for it is by structure alone 

 or structure and function combined that he is ena- 

 bled to build at all. But the impulse to build in 

 the large majority of cases is one that has so remote 

 a relation to the structure of the animal or his wants, 

 and his ability to build so far transcends what we 

 should expect from an examination of his structure, 

 that we could never tell beforehand how any ani- 

 mal would build. Nothing can well be more unlike 

 than the homes of animals that we should naturally 

 expect would build in the same manner. 



We see no tendency in the function of produc- 

 ing young even to originate the impulse to build or 

 to give the skill to build the numerous kinds of 

 nests found in the animal kingdom. In some cases 

 we see the need of the nests and dens if the young 

 are to come to maturity at all with any degree of 



