between 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 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:--

"Warum treibt sich das Volk so und schreit? Es will sich ernähren,
Kinder zeugen, und die nähren so gut es vermag.

* * * * *

Weiter bringt es kein Mensch, stell' er sich wie er auch will."[50]

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