236 A THEORY OF BIEDS' NESTS. 



peculiarity of structure which is hereditary; as when 

 the descendants of tumbler pigeons tumble, and the 

 descendants of pouter pigeons pout. In the present 

 case, however, I compare it strictly to the hereditary, 

 or more properly, persistent or imitative, habits of 

 savages, in building their houses as their fathers did. 

 Imitation is a lower faculty than invention. Children 

 and savages imitate before they originate ; birds, as 

 well as all other animals, do the same. 



The preceding observations are intended to show, 

 that the exact mode of nidification of each species of 

 bird is probably the result of a variety of causes, which 

 have been continually inducing changes in accordance 

 with changed organic or physical conditions. The 

 most important of these causes seem to be, in the first 

 place, the structure of the species, and, in the second, 

 its environment or conditions of existence. Now we 

 know, that every one of the characters or conditions 

 included under these two heads is variable. We have 

 seen that, on the large scale, the main features of the 

 nest built by each group of birds, bears a relation to 

 the organic structure of that group, and we have, 

 therefore, a right to infer, that as structure varies, the 

 nest will vary also in some particular corresponding 

 to the changes of structure. We have seen also, that 

 birds change the position, the form, and the con- 

 struction of their nest, whenever the available ma- 

 terials or the available situations, vary naturally or 

 have been altered by man; and we have, therefore, 

 a right to infer that similar changes have taken place, 



