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

228B THEK PHILOSOPHY OF BIRDS' SNETS.
buildings, and some rocks where these birds build, he
found many nests of the old type along with some of
the new pattern. He then examined all the figures
and descriptions of the older naturalists, and found
that they invariably represented the older form only.
The difference between the two forms he states to
be as follows. In the old form the nest is a portion of
a globe-when situated in the upper angle of a window
one-fourth of a hemisphere-and the opening is very
small and circular, being of a size just sufficient to allow
the body of the bird to pass. In the new form the
nest is much wider in proportion to its height, being
a segment of a depressed spheroid, and the aperture
is very wide and shallow, and close to the horizontal
surface to which the nest is attached above.
M. Pouchet thinks that the new form is an undoubted improvement on the old. The nest has a
wider bottom and must allow the young ones to have
more freedom of motion than in the old narrower, and
deeper nests, and its wide aperture allows the young
birds to peep out and breathe the fresh air. This is
so wide as to serve as a sort of balcony for them, and
two young ones can often be seen on it without interfering with the passage in and out of the old birds.
At the same time, by being so close to the roof, it is
a better protection against rain, against cold, and
against enemies, than the small round hole of the old
nests. Here, then, we have an improvement in nest
building, as well marked as any improvement that
takes place in human dwellings in so short a time.