m plasm is
thus simply handed on from one generation to the next.]

But plainly reproduction and heredity, as we have thus far considered
them, will be unable to account for the slow modification of the
machine; for in accordance with the facts thus far outlined, each
generation would be _precisely like the last_, and there would be no
chance for development and change from generation to generation. If the
individual is simply the unfolding of the powers possessed by a bit of
germ plasm, and if this germ plasm is simply handed on from generation
to generation, the successive generations must of necessity be
identical. But the living machine has been built by changes in the
successive generation, and hence plainly some other factor is needed.
This factor is _variation_.

==Variation.==--Variation is the principle that produces _modification of
type_. Heredity, as just explained, would make all generations alike.
But nothing is more certain than that they are not alike. The fact of
variation is patent on every side, for no two individuals are alike.
Successive generations differ from each other in one respect or
another. Birds vary in the length of their bills or toes; butterflies,
in their colours; dogs, in their size and shape and markings; and so on
through an endless category. Plants and animals alike throughout nature
show variations in the greatest profusion. It is these variations which
must furnish us with the foundation of the changes which have gradually
built up the living machine.

Of the fact of these variations there is no question, and the matter
need not detain us. Every one has had too many experiences to ask for
proof. Of the nature of the variations, however, there are some points
to be considered which are very germane to our subject. In the first
place, we must notice that these variations are of two kinds. There is
one class which is born with the individual, so that they are present
from the time of birth. In saying that these variations are born with
the individual we do not necessarily mean that they are externally
apparent at birth. A child may inherit from its parents characters which
do not appear till adult life. For example, a child may inherit the
colour of its father's hair, but this colour is not apparent at birth.
It appears only in later life, but it is none the less an inborn
character. In the same way, we may have many inborn variations among
individuals which do not make themselves