temperature of the air is insufficient to effect this, she sits upon
her eggs, and only intermits her sittings in the warmest countries;
the mammal, on the other hand, attains the fulfilment of its
instinctive purpose without any co-operation on its own part. In
warm climates many birds only sit by night, and small exotic birds
that have built in aviaries kept at a high temperature sit little
upon their eggs or not at all. How inconceivable is the supposition
of a mechanism that impels the bird to sit as soon as the temperature
falls below a certain height! How clear and simple, on the other
hand, is the view that there is an unconscious purpose constraining
the volition of the bird to the use of the fitting means, of which
process, however, only the last link, that is to say, the will
immediately preceding the action falls within the consciousness of
the bird!

In South Africa the sparrow surrounds her nest with thorns as a
defence against apes and serpents. The eggs of the cuckoo, as
regards size, colour, and marking, invariably resemble those of the
birds in whose nests she lays. Sylvia ruja, for example, lays a
white egg with violet spots; Sylvia hippolais, a red one with black
spots; Regulus ignicapellus, a cloudy red; but the cuckoo's egg is in
each case so deceptive an imitation of its model, that it can hardly
be distinguished except by the structure of its shell.

Huber contrived that his bees should be unable to build in their
usual instinctive manner, beginning from above and working downwards;
on this they began building from below, and again horizontally. The
outermost cells that spring from the top of the hive or abut against
its sides are not hexagonal, but pentagonal, so as to gain in
strength, being attached with one base instead of two sides. In