It
lengthens legs because they are used in supporting the body, and
shortens arms because they are used in pulling. Whether it enlarges
brain if used in one way and diminishes it if used in another, we cannot
tell; but it must obviously deaden nervous sensibilities in some cases
and intensify them in others. It enlarges hands long before they are
used, and thickens soles long before the time for walking on them. At
the same time, as if by an oversight, it so delays its transmission of
the habit of walking on these thickened soles, that the gradual and
tedious acquisition of the non-transmitted habit costs the infant much
time and trouble and often some pain and danger. Yet where aided by
natural selection, as with chickens and foals, it transmits the habit in
wonderful perfection and at a remarkably early date. It transmits new
paces in horses in a single generation, but fails to perpetuate the
songs of birds. It modifies offspring like parents, and yet allows the
formation of two reproductive types in plants, and of two or more types
widely different from the parents in some of the higher insects. It is
said to be indispensable for the co-ordinated development of man and the
giraffe and the elk, but appears to be unnecessary for the evolution and
the maintenance of wonderful structures and habits and instincts in a
thousand species of ants and bees and termites. It is the only possible
means of complex evolution and adaptation of co-operative parts, and yet
in Mr. Spencer's most representative case it renders such important
parts as teeth and jaws unsuited for each other, and is said to ruin the
teeth by the consequent overcrowding and decay. It survives amidst a
general "lack of recognised evidence," and only seems to act usefully
and healthily and regularly in quarters where it can least easily be
distinguished from other more powerful and demonstrable factors of
evolution. So little does it care to display its powers where they would
be easily verifiable as well as useful that practical breeders ignore
it. So slight is its independent power that it seems to allow natural
selection or sexual selection or artificial selection to modify
organisms in sheer defiance of its utmost opposition, just as readily as
they modify organisms in other directions with its utmost help. If it
partially perpetuates and extends the pecked-out indentations in the
motmot's tail feathers, it on the other hand fails to transmit the
slightest