ust become distinguishable from the egg in
which it arises, it is nothing but an aggregation of such corpuscles,
and every organ of the body was, once, no more than such an aggregation.

Thus a nucleated mass of protoplasm turns out to be what may be termed
the structural unit of the human body. As a matter of fact, the body, in
its earliest state, is a mere multiple of such units; and in its perfect
condition, it is a multiple of such units, variously modified.

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.
Traced back to its earliest state, the nettle arises as the man does, in
a particle of nucleated protoplasm. And in the lowest plants, as in
the lowest animals, a single mass of such protoplasm may constitute the
whole plant, or the protoplasm may exist without a nucleus.

Under these circumstances it may well be asked, how is one mass of
non-nucleated protoplasm to be distinguished from anoth