122 HERBERT SPENCER

units of chromatin, ever undergoing changes, diffuse energy
around, they may also be units which, under the conditions
furnished by fertilisation, gravitate towards the organisation
of the species. Possibly it may be that the complex com-
bination of proteids, common to chromatin and cytoplasm, is
that part in which constitutional characters inhere; while the
pjb^horffled component, falling from its unstable union and
decomposing, evolves the energy which, ordinarily the cause
of changes, now excites the more active changes following
fertilisation."

From this speculation Spencer passes to a brief considera-
tion of what occurs before and during the fertilisation of the
ovum. Before fertilisation is accomplished the nucleus of
the ovum normally divides twice in rapid succession, and
gives off two abortive cells—known as polar bodies—which
come to nothing. The usual result of this "maturation,"
as it is called, is that the number of chromosomes in the
ovum is reduced to a half of the normal number characteristic
of the cells of the species to which it belongs. In the
history of the male element or spermatozoon, there is an
analogous reduction, so that when spermatozoon and ovum
unite in fertilisation the normal number is restored. It is
now recognised that the maturation-divisions are useful
in obviating the doubling of the number of chromosomes
which fertilisation would otherwise involve, and it has also
been suggested that this continually recurrent elimination of
chromosomes may be one of the causes of variation.

Spencer suggested another interpretation. He pointed out
the general fact that sexual reproduction (gamogenesis)
commonly occurs when asexual reproduction (agamogenesis)
is arrested by unfavourable conditions, that failing asexual
reproduction initiates sexual reproduction. Now as egg-cells
and sperm-cells are the outcome of often long series of cell
divisions (asexual multiplication), may not the polar bodies,
which are aborted cells, indicate that asexual multiplication
can no longer go on, and that the conditions leading to sexual
multiplication have set in ? " As the cells which become
spermatozoa are left with half the number of chromosomes
possessed by preceding cells, there is actually that impoverish-
ment and declining vigour here suggested as the antecedent