and decaying vegetable matter, as much as would represent several
cartloads of mould; and in this natural hot-bed the hens lay their
eggs, burying each separately with a good stock of leaves around it.
The heat of the sun and the fermenting mould hatch them out between
them; to expedite the process, the birds uncover the eggs during the
warmer part of the day, expose them to the sun, and bury them again in
the hot-beds towards evening. Several intermediate steps may also be
found between this early stage of communal nesting by proxy and the
true hatching instinct; a good one is supplied by the ostrich, which
partially buries its eggs in hot sand, but sits on them at intervals,
both father and mother birds taking shares by turn in the duties of
incubation.

The vast subject which I have thus lightly skimmed is not without
interest, again, from its human implications. Savages as a rule
produce enormous families; but then, the infant mortality in savage
tribes is proportionally great. Among civilized races, families are
smaller, and deaths in infancy are far less numerous. The higher the
class or the natural grade of a stock, the larger as a rule the
proportion of children safely reared to the adult age. The goal
towards which humanity is slowly moving would thus seem to be one
where families in most cases will be relatively small--perhaps not
more on an average than three to a household--but where most or all of
the children brought into the world will be safely reared to full
maturity. This is already becoming the rule in certain favored ranks
of European society.

[Illustration: TADPOLES AND YOUNG FROG.]