our own mobs of sheep careful culling tends to improve the flock.

C. This, too, is obvious.

F. Extend this to all animals and plants, and the same thing will
hold good concerning them all. I shall now change the ground and
demand assent to another statement. You know that though the
offspring of all plants and animals is in the main like the parent,
yet that in almost every instance slight deviations occur, and that
sometimes there is even considerable divergence from the parent type.
It must also be admitted that these slight variations are often, or
at least sometimes, capable of being perpetuated by inheritance.
Indeed, it is only in consequence of this fact that our sheep and
cattle have been capable of so much improvement.

C. I admit this.

F. Then the whole matter lies in a nutshell. Suppose that hundreds
of millions of years ago there existed upon this earth a single
primordial form of the very lowest life, or suppose that three or
four such primordial forms existed. Change of climate, of food, of
any of the circumstances which surrounded any member of this first
and lowest class of life would tend to alter it in some slight
manner, and the alteration would have a tendency to perpetuate itself
by inheritance. Many failures would doubtless occur, but with the
lapse of time slight deviations would undoubtedly become permanent
and inheritable, those alone being perpetuated which were beneficial
to individuals in whom they appeared. Repeat the process with each
deviation and we shall again obtain divergences (in the course of
ages) differing more strongly from the ancestral form, and again