236 A THEORY OF BIRDS’ 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, 
