se
apposition by means of other cells and binding membranes,--with the
result of giving rise to those various "tissues," which in turn go to
constitute the material of "organs." I cannot suppose, however, that any
advocate of discontinuity will care to take his stand at this point.
But, if any one were so foolish as to do so, it would be easy to
dislodge him by describing the state of matters in some of the Protozoa
where a number of unicellular "individuals" are organically united so as
to form a "colony." These cases serve to bridge this distinction between
Protozoa and Metazoa, of which therefore we may now take leave.

In the second place, there is the no less obvious distinction that the
result of cell-division in the Metazoa is not merely to multiply cells
all of the same kind: on the contrary, the process here gives rise to as
many different kinds of cells as there are different kinds of tissue
composing the adult organism. But no one, I should think, is likely to
oppose the doctrine of continuity on the ground of this distinction. For
the distinction is clearly one which must necessarily arise, if the
doctrine of continuity between unicellular and multicellular organisms
be true. In other words, it is a distinction which the theory of
evolution itself must necessarily pre-suppose, and therefore it is no
objection to the theory that its pre-supposition is realized. Moreover,
as we shall see better presently, there is no difficulty in
understanding why this distinction should have arisen, so soon as it
became necessary (or desirable) that individual cells, when composing a
"colony," should conform to the economic principle of the division of
labour--a principle, indeed, which is already foreshadowed in the
constituent parts of a single cell, since the nucleus has one set of
functions and its surrounding protoplasm another.

But now, in the third place, we arrive at a more important distinction,
and one which lies at the root of the others still remaining to be
considered. I refer to sexual propagation. For it is a peculiarity of
the multicellular organisms that, although many of them may likewise
propagate themselves by other means (Fig. 28), they all propagate
themselves by means of sexual congress. Now, in its essence, sexual
congress consists in the fusion of two specialized cells (or, as now
seems almost certain, of the nuclei thereof), so that it is out of such
a combination that the new individual arises by