erely 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, 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 construction of their
nest, whenever the available materials or the ava