tantly close together[2] in
virtue of an act of contraction of part of their substance, just as the
body of a snail contracts into its shell when one of its "horns" is
irritated.

[Footnote 2: Darwin, _Insectivorous Plants_, p. 289.]

The reflex action of the snail is the result of the presence of a nervous
system in the animal. A molecular change takes place in the nerve of the
tentacle, is propagated to the muscles by which the body is retracted,
and causing them to contract, the act of retraction is brought about. Of
course the similarity of the acts does not necessarily involve the
conclusion that the mechanism by which they are effected is the same; but
it suggests a suspicion of their identity which needs careful testing.

The results of recent inquiries into the structure of the nervous system
of animals converge towards the conclusion that the nerve fibres, which
we have hitherto regarded as ultimate elements of nervous tissue, are not
such, but are simply the visible aggregations of vastly more attenuated
filaments, the diameter of which dwindles down to the limits of our
present microscopic vision, greatly as these have been extended by modern
improvements of the microscope; and that a nerve is, in its essence,
nothing but a linear tract of specially modified protoplasm between two
points of an organism--one of which is able to affect the other by means
of the communication so established. Hence, it is conceivable that even
the simplest living being may possess a nervous system. And the question
whether plants are provided with a nervous system or not, thus acquires a
new aspect, and presents the histologist and physiologist with a problem
of extreme difficulty, which must be attacked from a new point of view
and by the aid of methods which have yet to be invented.

Thus it must be admitted that plants may be contractile and locomotive;
that, while locomotive, their movements may have as much appearance of
spontaneity as those of the lowest animals; and that many exhibit
actions, comparable to those which are brought about by the agency of a
nervous system in animals. And it must be allowed to be possible that
further research may reveal the existence of something comparable to a
nervous system in plants. So that I know not where we can hope to find
any absolute distinction between animals and plants, unless we return to
their mode of nutrition, and inquire whether certain differences of a
more occult chara