atory process.]

Thus, by the progress of knowledge, Cuvier's fourth distinction between
the animal and the plant has been as completely invalidated as the third
and second; and even the first can be retained only in a modified form
and subject to exceptions.

But has the advance of biology simply tended to break down old
distinctions, without establishing new ones?

With a qualification, to be considered presently, the answer to this
question is undoubtedly in the affirmative. The famous researches of
Schwann and Schleiden in 1837 and the following years, founded the modern
science of histology, or that branch of anatomy which deals with the
ultimate visible structure of organisms, as revealed by the microscope;
and, from that day to this, the rapid improvement of methods of
investigation, and the energy of a host of accurate observers, have given
greater and greater breadth and firmness to Schwann's great
generalisation, that a fundamental unity of structure obtains in animals
and plants; and that, however diverse may be the fabrics, or _tissues_,
of which their bodies are composed, all these varied structures result
from the metamorphosis of morphological units (termed _cells_, in a more
general sense than that in which the word "cells" was at first employed),
which are not only similar in animals and in plants respectively, but
present a close resemblance, when those of animals and those of plants
are compared together.

The contractility which is the fundamental condition of locomotion, has
not only been discovered to exist far more widely among plants than was
formerly imagined; but, in plants, the act of contraction has been found
to be accompanied, as Dr. Burdon Sanderson's interesting investigations
have shown, by a disturbance of the electrical state of the contractile
substance, comparable to that which was found by Du Bois Reymond to be a
concomitant of the activity of ordinary muscle in animals.

Again, I know of no test by which the reaction of the leaves of the
Sundew and of other plants to stimuli, so fully and carefully studied by
Mr. Darwin, can be distinguished from those acts of contraction following
upon stimuli, which are called "reflex" in animals.

On each lobe of the bilobed leaf of Venus's fly-trap (_Dionoea
muscipula_) are three delicate filaments which stand out at right angle
from the surface of the leaf. Touch one of them with the end of a fine
human hair and the lobes of the leaf ins