Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

A THEORY OF BIRDS' NESTS. 237
when, by a natural process, external conditions have
become in any way permanently altered. We must
remember, however, that all these factors are very
stable during many generations, and only change at
a rate commensurate with those of the great physical
features of the earth as revealed to us by geology; and
we may, therefore, infer that the form and construction
of nests, which we have shown to be dependent on
them, are equally stable. If, therefore, we find less
important and more easily modified characters than
these, so correlated with peculiarities of nidification as
to indicate that one is probably the cause of the
other, we shall be justified in concluding that these
variable characters are dependent on the mode of nidification, and not that the form of the nest has been
determined by these variable characters. Such a correlation I am now about to point out.
Classification of Nests.
For the purpose of this inquiry it is necessary to
group nests into two great classes, without any regard to their most obvious differences or resemblances,
but solely looking to the fact of whether the contents
(eggs, young, or sitting bird) are hidden or exposed
to view. In the first class we place all those in
which the eggs and young are completely hidden, no
matter whether this is effected by an elaborate covered
structure, or by depositing the eggs in some hollow
tree or burrow underground. In the second, we group
all in which the eggs, young, and sitting bird are