living body, which have
hardly yet received all the development of which they are capable.

Kant defined the special peculiarity of the living body to be that the
parts exist for the sake of the whole and the whole for the sake of the
parts. But when Turpin and Schwann resolved the living body into an
aggregation of quasi-independent cells, each, like a _Torula_, leading
its own life and having its own laws of growth and development, the
aggregation being dominated and kept working towards a definite end only
by a certain harmony among these units, or by the superaddition of a
controlling apparatus, such as a nervous system, this conception ceased
to be tenable. The cell lives for its own sake, as well as for the sake
of the whole organism; and the cells which float in the blood, live at
its expense, and profoundly modify it, are almost as much independent
organisms as the _Toruloe_ which float in beer-wort.

Schwann burdened his enunciation of the "cell theory" with two false
suppositions; the one, that the structures he called "nucleus"[6] and
"cell-wall" are essential to a cell; the other, that cells are usually
formed independently of other cells; but, in 1839, it was a vast and
clear gain to arrive at the conception, that the vital functions of all
the higher animals and plants are the resultant of the forces inherent in
the innumerable minute cells of which they are composed, and that each of
them is, itself, an equivalent of one of the lowest and simplest of
independent living beings--the _Torula_.

[Footnote 6: Later investigations have thrown an entirely new light upon
the structure and the functional importance of the nucleus; and have
proved that Schwann did not over-estimate its importance. 1894.]

From purely morphological investigations, Turpin and Schwann, as we have
seen, arrived at the notion of the fundamental unity of structure of
living beings. And, before long, the researches of chemists gradually led
up to the conception of the fundamental unity of their composition.

So far back as 1803, Thenard pointed out, in most distinct terms, the
important fact that yeast contains a nitrogenous "animal" substance; and
that such a substance is contained in all ferments. Before him, Fabroni
and Fourcroy speak of the "vegeto-animal" matter of yeast. In 1844 Mulder
endeavoured to demonstrate that a peculiar substance, which he called
"protein," was essentially characteristic of living matter.

In 1846, Paye