at in the
majority of animals, and also in many plants, an individual life
begins in the union of two minute elements, the mother egg-cell and
the sperm father-cell. But this is not the earliest stage, and below
these higher forms we find a great world of life reproducing without
this sex-process by simple separation and growth. In these unicellular
organisms reproduction is known as asexual, because there are no
special germ-cells, nor is there anything corresponding to
fertilisation. The most common forms are (1) by division into two; (2)
by budding, a modified form of division; (3) by sporulation, a
division into many units.[9]

It is worth while to wait to learn something of this first stage in
the development of life, for in this way we shall gain a clue as to
the origin of sex and the real purpose it fulfils in the service of
reproduction. In the very simplest forms of unicellular organisms
propagation is effected at what is known as "the limit of growth";
when the cell has attained as much volume as its surface can
adequately supply with food, a simple division of the cell takes place
into two halves or daughter cells, each exactly like the other, which
then become independent and themselves repeat the same rupture
process. But in some slightly more complex cases differences occur
between the two cells into which the organism divides, as in the
_slipper animacule_, where one-half goes off with the mouth, while the
other has none. In a short time, however, the mouthless half forms a
mouth, and each half grows into a replica of the original. We have
here one of the earliest examples of differentiation. That injured
multicellular organisms should be able by regrowth to repair their
loss in an analogous phenomenon; thus an earth-worm cut by a spade
does not necessarily suffer loss, but the head part grows a tail and
the decapitated portion produces a head; sponges, which do not
normally propagate by division, may be cut in pieces and bedded out
successfully; the arms of a star-fish, torn asunder by a fisherman,
will almost always result in several perfect star-fish. Similarly
among plants a cut-off portion may readily give rise to new plants--a
potato-tuber is one of hundreds of instances. This ability to effect
complete repair is one of the powers that life has lost; it persists
as high in the scale as reptiles, and a lizard is able to regrow an
amputated leg.

It is certainly not the least interest in studying t