s the animal to itself. But Nature never
leaves the animal to itself. Her changes are slight and slow; but they
never go back. They are permanent, only to be reaffected as the
environment alters. When we consider the incalculable, inconceivable
lapse of time through which organic life has been swayed by the
never-ceasing action of the forces around it, we can imagine what a vast
variety of animal forms may have been evolved from some one primal
ancestor.

Darwin endeavors to explain, in detail, how this differentiation takes
place. The largest or strongest get the best food or the most attractive
females, and then transmit their strength or their peculiarities to
their progeny. These peculiarities are the results of the environment,
and if this shall go on changing in the direction of these
peculiarities, they will increase. Suppose that the climate should
gradually grow colder; the result might be a denser growth of hair.
Those which, by strength or otherwise, chanced to have denser hair than
others, would more readily endure the change, and would transmit the
habit to their young; others less fitted to endure the change would die
out. In a short time the young would be born with the dense hair, as it
is well known that any new habit assumed by animals is exhibited earlier
and earlier in the young, as long as it is a necessity of life. These
variations are never due to one single cause. Organized life is wrought
upon by a wonderfully complex web of influences. Darwin has the
following passage touching the action of vegetable and animal life upon
each other.

"In Staffordshire, on the estate of a relation where I had ample means
of investigation, there was a large and extremely barren heath, which
had never been touched by the hand of man; but several hundred acres of
exactly the same nature had been enclosed twenty-five years previously,
and planted with Scotch fir. The change in the native vegetation of the
planted part of the heath was most remarkable,--more than is generally
seen in passing from one quite different soil to another; not only the
proportional numbers of the heath plants were wholly changed, but twelve
species of plants (not counting grasses and carices) flourished in the
plantations, which could not be found on the heath. The effect on the
insects must have been still greater, for six insectivorous birds were
very common in the plantations which were not to be seen on the heath;
and the heath was fr