continuous force which binds age to age, which enables each to begin
with some improvement on the last, if the last did itself improve;
which makes each civilisation not a set of detached dots, but a line
of colour, surely enhancing shade by shade. There is, by this
doctrine, a physical cause of improvement from generation to
generation: and no imagination which has apprehended it can forget
it; but unless you appreciate that cause in its subtle materialism,
unless you see it, as it were, playing upon the nerves of men, and,
age after age, making nicer music from finer chords, you cannot
comprehend the principle of inheritance either in its mystery or its
power.

These principles are quite independent of any theory as to the
nature of matter, or the nature of mind. They are as true upon the
theory that mind acts on matter--though separate and altogether
different from it--as upon the theory of Bishop Berkeley that there
is no matter, but only mind; or upon the contrary theory--that there
is no mind, but only matter; or upon the yet subtler theory now
often held--that both mind and matter are different modifications of
some one tertium quid, some hidden thing or force. All these
theories admit--indeed they are but various theories to account for-
-the fact that what we call matter has consequences in what we call
mind, and that what we call mind produces results in what we call
matter; and the doctrines I quote assume only that. Our mind in some
strange way acts on our nerves, and our nerves in some equally
strange way store up the consequences, and somehow the result, as a
rule and commonly enough, goes down to our descendants; these
primitive facts all theories admit, and all of them labour to
explain.