rtebra hence resulting (as in some
pigeons) proves beneficial, survival of the fittest may make it a
constant character; and there may, by further like additions, be
produced extremely long strings of vertebrae, such as snakes show us.
Similarly with the mammary glands. It is not an unreasonable supposition
that by the effects of greater or less function, inherited through
successive generations, these may be enlarged or diminished in size; but
it is out of the question to allege such a cause for changes in their
numbers. There is no imaginable explanation of these save the
establishment by inheritance of spontaneous variations, such as are
known to occur in the human race.

So too, in the third place, with certain alterations in the connexions
of parts. According to the greater or smaller demands made on this or
that limb, the muscles moving it may be augmented or diminished in bulk;
and, if there is inheritance of changes so wrought, the limb may, in
course of generations, be rendered larger or smaller. But changes in the
arrangements or attachments of muscles cannot be thus accounted for. It
is found, especially at the extremities, that the relations of tendons
to bones and to one another are not always the same. Variations in their
modes of connexion may occasionally prove advantageous, and may thus
become established. Here again, then, we have a class of structural
changes to which Mr. Darwin's hypothesis gives us the key, and to which
there is no other key.

Once more there are the phenomena of mimicry. Perhaps in a more striking
way than any others, these show how traits which seem inexplicable are
explicable as due to the more frequent survival of individuals that have
varied in favourable ways. We are enabled to understand such marvellous
simulations as those of the leaf-insect, those of beetles which
"resemble glittering dew-drops upon the leaves;" those of caterpillars
which, when asleep, stretch themselves out so as to look like twigs. And
we are shown how there have arisen still more astonishing
imitations--those of one insect by another. As Mr. Bates has proved,
there are cases in which a species of butterfly, rendered so unpalatable
to insectivorous birds by its disagreeable taste that they will not
catch it, is simulated in its colours and markings by a species which is
structurally quite different--so simulated that even a practised
entomologist is liable to be deceived: the explanation being that an