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]

We are now in a position to gain a clue to the difficult problem of
the origin of the sexes. It would be easy as well as instructive to
accumulate examples.[14] I am tempted to linger over the
life-histories of these early organisms that are so full of
suggestion; but the case I have selected--the _volvox_--really answers
the question. Sex here is dependent on, and would seem to have arisen
through, differences in environmental conditions. We find the
well-nourished, larger, and usually more quiescent cell is the female,
the hungrier and more mobile cell the male; the one concerned with
storing energy, the other with consuming it, the one building up, the
other breaking down; or expressed in biological formula, the female
cell is predominantly anabolic, that of the male predominantly
katabolic. Thus we find that the male, through a want of nutrition,
was carried developmentally away from the well-fed female cell, which
it was bound to seek and unite with to continue life. This relation
between the food supply and the sexes is found persisting in higher
forms, and, in this connection, the well-known experiments of Young on
tadpoles and of Siebald on wasps may be cited. By increasing the
nutrition of tadpoles the percentage of females was raised from the
normal of about fifty per cent. to ninety, while similarly among wasps
the number of females was found to depend on warmth and food supply,
and to decrease as these diminished. Mention also may be made of the
plant-lice, or aphides, which infest our rose-bushes and other plants,
which, during the summer months, when conditions are favourable,
produce generation after generation of females, but on the advent of
autumn, with its cold and scarcity of food, males appear and sexual
reproduction takes place. Similarly brine-shrimps when living under
favourable conditions produce females, but when the environment is
less favourable males as w