ng filament, endued indeed with
different kinds of irritabilities and sensibilities, or of animal
appetencies; which exist in every gland, and in every moving organ of the
body, and are as essential to living organization as chemical affinities
are to certain combinations of inanimate matter.

If I might be indulged to make a simile in a philosophical work, I should
say, that the animal appetencies are not only perhaps less numerous
originally than the chemical affinities; but that like these latter, they
change with every new combination; thus vital air and azote, when combined,
produce nitrous acid; which now acquires the property of dissolving silver;
so with every new additional part to the embryon, as of the throat or
lungs, I suppose a new animal appetency to be produced.

In this early formation of the embryon from the irritabilities,
sensibilities, and associabilities, and consequent appetencies, the faculty
of volition can scarcely be supposed to have had its birth. For about what
can the fetus deliberate, when it has no choice of objects? But in the more
advanced state of the fetus, it evidently possesses volition; as it
frequently changes its attitude, though it seems to sleep the greatest part
of its time; and afterwards the power of volition contributes to change or
alter many parts of the body during its growth to manhood, by our early
modes of exertion in the various departments of life. All these faculties
then constitute the vis fabricatrix, and the vis conservatrix, as well as
the vis medicatrix of nature, so much spoken of, but so little understood
by philosophers.

8. When we revolve in our minds, first, the great changes, which we see
naturally produced in animals after their nativity, as in the production of
the butterfly with painted wings from the crawling caterpillar; or of the
respiring frog from the subnatant tadpole; from the feminine boy to the
bearded man, and from the infant girl to the lactescent woman; both which
changes may be prevented by certain mutilations of the glands necessary to
reproduction.

Secondly, when we think over the great changes introduced into various
animals by artificial or accidental cultivation, as in horses, which we
have exercised for the different purposes of strength or swiftness, in
carrying burthens or in running races; or in dogs, which have been
cultivated for strength and courage, as the bull-dog; or for acuteness of
his sense or smell, as the hound an