means, that all the multifarious and
complicated activities of man are comprehensible under three categories.
Either they are immediately directed towards the maintenance and
development of the body, or they effect transitory changes in the
relative positions of parts of the body, or they tend towards the
continuance of the species. Even those manifestions of intellect, of
feeling, and of will, which we rightly name the higher faculties, are
not excluded from this classification, inasmuch as to every one but the
subject of them, they are known only as transitory changes in the
relative positions of parts of the body. Speech, gesture, and every
other form of human action are, in the long run, resolvable into
muscular contraction, and muscular contraction is but a transitory
change in the relative positions of the parts of a muscle. But the
scheme which is large enough to embrace the activities of the highest
form of life, covers all those of the lower creatures. The lowest plant,
or animalcule, feeds, grows, and reproduces its kind. In addition, all
animals manifest those transitory changes of form which we class under
irritability and contractility; and it is more than probable that when
the vegetable world is thoroughly explored, we shall find all plants in
possession of the same powers, at one time or other of their existence.

I am not now alluding to such phenomena, at once rare and conspicuous,
as those exhibited by the leaflets of the sensitive plants, or the
stamens of the barberry, but to much more widely spread, and at the same
time, more subtle and hidden, manifestions of vegetable contractility.
You are doubtless aware that the common nettle owes its stinging
property to the innumerable stiff and needle-like, though exquisitely
delicate, hairs which cover its surface. Each stinging-needle tapers
from a broad base to a slender summit, which, though rounded at the end,
is of such microscopic fineness that it readily penetrates, and breaks
off in, the skin. The whole hair consists of a very delicate outer case
of wood, closely applied to the inner surface of which is a layer of
semi-fluid matter, full of innumerable granules of extreme minuteness.
This semi-fluid lining is protoplasm, which thus constitutes a kind of
bag, full of a limpid liquid, and roughly corresponding in form with the
interior of the hair which it fills. When viewed with a sufficiently
high magnifying power, the protoplasmic layer of the nett