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 seen accumulating nourishment at the expense of the others and grow larger, and if this continues, cells which must be regarded as ova, or female cells, result; while other cells, less advantageously placed with more competitors struggling to obtain food, grow smaller and gradually change their character, becoming, in fact, males. In some cases distinct colonies may in this way arise, some composed entirely of the large well-nourished cells, and others of small hungry cells, and may be recognised as completely female or male colonies.[13]