it was the improving and perfecting of the single-cell
process by introducing variation through the commingling of the
ancestral hereditary elements of two parents, and, by means of such
variations, the production of new and higher forms of life--in fact,
progress by the mighty dynamic of sex.[12]

As we should expect, the passing from the sexless mode of reproduction
to the definite male and female types is not sharply defined or
abrupt. Even among many unicellular organisms the process becomes more
elaborate with distinct specialisation of reproductive elements. In
some cases conjugation is observed, when two individuals coalesce, and
each cell and each nucleus divides into two, and each half unites with
the half of the other to form a new cell. This is asexual, since the
uniting cells are exactly similar, but the effect would seem to be the
strengthening of the cells by, as it were, introducing new blood. In
somewhat more complex cases these cells do not part company when they
divide, but remain attached to one another, and form a kind of
commonwealth. Here one can see at once that some cells in a little
group will be less advantageously placed for the absorption of
nourishment than others. By degrees this differentiation of function
brings about differentiation of form, and cells become modified, in
some cases, to a surprising extent, to serve special purposes. The
next advance is when the uniting cells become somewhat different in
themselves. In the early stages this difference appears as one of
size; a small weakly cell, though sometimes propagating by union with
a similar cell, in other cases seeks out a larger and more developed
cell, and by uniting with it in mutual nourishment becomes strong.
This may be seen among the _protozoa_ where we can trace the distinct
beginnings of the male and female elements. A very instructive example
is furnished by the case of _volvox_, a multicellular vegative
organism of very curious habits. The cells at first are all alike;
they are united by protoplasmic bridges and form a colony. In
favourable environmental conditions of abundant nutrition this state
of affairs continues, and the colony increases only by multiplication
and without fertilisation. But when the supply of food is exhausted,
or by any cause is checked, sexual reproduction is resorted to, and
this in a way that illustrates most instructively the differentiation
of the female and male cells. Some of the cells are