wards 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 manifestations 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 phaenomena, at once rare and conspicuous,
as those exhibited by the leaflets of the sensitive plant, or the
stamens of the barberry, but to much more widely-spread, and, at the
same time, more subtle and hidden, manifestations 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 semifluid 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 nettle hair is seen to be in a condition of
unceasing activity. Local contractions of the whole thickness of its
substance pass slowly and gradual