een
the dense and resisting mass of the oak, or the strong fabric 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 everyone 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 the powers of mankind into the
well-known epigram:--

Warum treibt sich das Volk so und schreit? Es will sich ernaehren,
Kinder zeugen, und die naehren 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 toward the maintenance and
development of the body, or they effect transitory changes in the relative
positions of parts of the body, or they tend toward 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 everyone 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