abric
of the tortoise, and those broad disks of glassy jelly which may be seen
pulsating through the waters of a calm sea, but which drain away to mere
films in the hand which raises them out of their element?

Such objections as these must, I think, arise in the mind of every
one who ponders, for the first time, upon the conception of a single
physical basis of life underlying all the diversities of vital
existence; but I propose to demonstrate to you that, notwithstanding
these apparent difficulties, a threefold unity--namely, a unity of
power or faculty, a unity of form, and a unity of substantial
composition--does pervade the whole living world.

No very abstruse argumentation is needed, in the first place to prove
that the powers, or faculties, of all kinds of living matter, diverse as
they may be in degree, are substantially similar in kind.

Goethe has condensed a survey of all powers of mankind into the
well-known epigram:--[95]

"Warum treibt sich das Volk so und schreit? Es will sich ernahren Kinder
zeugen, und die nahren so gut es vermag. . . . . . . . . . . . . .
Weiter bringt es kein Mensch, stell' er sich wie er auch will."

In physiological language this 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 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 fi