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 labor to explain.
