VI A THEORY OF BIRDS’ NESTS 121 
their elders, without any regard to the use or applicability of 
the particular acts. So, in savages, many customs peculiar to 
each tribe are handed down from father to son merely by the 
force of habit, and are continued long after the purpose which 
they originally served has ceased to exist. With these and a 
hundred similar facts everywhere around us, we may fairly 
impute much of what we cannot understand in the details of 
Bird-Architecture to an analogous cause. If we do not do so, 
we must assume either that birds are guided in every action 
by pure reason to a far greater extent than men are, or that 
an infallible instinct leads them to the same result by a 
different road. The first theory has never, that I am aware 
of, been maintained by any author, and I have already shown 
that the second, although constantly assumed, has never been 
proved, and that a large body of facts is entirely opposed to 
it. One of my critics has, indeed, maintained that I admit 
“instinct” under the term “hereditary habit”; but the 
whole course of my argument shows that I do not do so. 
Hereditary habit is, indeed, the same as instinct when the 
term is applied to some simple action dependent upon a 
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, per- 
sistent 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 
