But does the formula which expresses the essential structural character
of the highest animal cover all the rest, as the statement of its powers
and faculties covered that of all others? Very nearly. Beast and
fowl, reptile and fish, mollusk, worm, and polype, are all composed of
structural units of the same character, namely, masses of protoplasm
with a nucleus. There are sundry very low animals, each of which,
structurally, is a mere colourless blood-corpuscle, leading an
independent life. But, at the very bottom of the animal scale, even
this simplicity becomes simplified, and all the phaenomena of life are
manifested by a particle of protoplasm without a nucleus. Nor are such
organisms insignificant by reason of their want of complexity. It is a
fair question whether the protoplasm of those simplest forms of life,
which people an immense extent of the bottom of the sea, would not
outweigh that of all the higher living beings which inhabit the land put
together. And in ancient times, no less than at the present day, such
living beings as these have been the greatest of rock builders.
What has been said of the animal world is no less true of plants.
Imbedded in the protoplasm at the broad, or attached, end of the nettle
hair, there lies a spheroidal nucleus. Careful examination further
proves that the whole substance of the nettle is made up of a repetition
of such masses of nucleated protoplasm, each contained in a wooden case,
which is modified in form, sometimes into a woody fibre, sometimes into
a duct or spiral vessel, sometimes into a pollen grain, or an ovule.
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