ilable situations, vary
naturally or have been altered by man; and we have, therefore, a right
to infer that similar changes have taken place, 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 exposed to view, no matter whether there is
the most beautifully formed nest, or none at all. Kingfishers, which
build almost invariably in holes in banks; Woodpeckers and Parrots,
which build in hollow trees; the Icteridae of America, which all make
beautiful covered and suspended nests; and our own Wren, which builds a
domed nest, are examples of the former; while our Thrushes, Warblers,
and Finches, as well as the Crowshrikes, Chatterers, and Tanagers of the
tropics, together with all Raptorial birds and Pigeons, and a vast
number of others in every part of the world, all adopt the latter mode
of building.

It will be seen that this division of birds according to their
nidification, bears little relation to the character of the nest itself.
It is a functional not a structural classification. The most rude and
the m