res come and
go around its vast circumference? Or, turning to the other half of the
world of life, picture to yourselves the great Finner whale, hugest of
beasts that live, or have lived, disporting his eighty or ninety feet of
bone, muscle, and blubber, with easy roll, among waves in which the
stoutest ship that ever left dockyard would founder hopelessly; and
contrast him with the invisible animalcules--mere gelatinous specks,
multitudes of which could, in fact, dance upon the point of a needle
with the same ease as the angels of the Schoolmen could, in imagination.
With these images before your minds, you may well ask, what community
of form, or structure, is there between the animalcule and the whale; or
between the fungus and the fig-tree? And, _a fortiori_, between all
four?

Finally, if we regard substance, or material composition, what hidden
bond can connect the flower which a girl wears in her hair and the blood
which courses through her youthful veins; or, what is there in common
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 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 to