tion
is universal, and is based upon that harmonious antagonism between the
sexes, that contrast between the male and the female element, that at
once divides and unites the whole Animal Kingdom. And although this
exchange of influence is not kept up by an equality of numeric
relations,--since not only are the sexes very unequally divided in some
kinds of animals, but the male and female elements are even combined
in certain types, so that the individuals are uniformly
hermaphrodites,--yet I firmly believe that this numerical distribution,
however unequal it may seem to us, is not without its ordained accuracy
and balance. He who has assigned its place to every leaf in the thickest
forest, according to an arithmetical law which prescribes to each its
allotted share of room on the branch where it grows, will not have
distributed animal life with less care.

But although reproduction by eggs is common to all animals, it is only
one among several modes of multiplication. We have seen that certain
animals, besides the ordinary process of generation, also increase their
number naturally and constantly by self-division, so that out of one
individual many individuals may arise by a natural breaking up of
the whole body into distinct surviving parts. This process of normal
self-division may take place at all periods of life: it may form an
early phase of metamorphosis, as in the Hydroid of our common Aurelia,
described in the last article; or it may even take place before the
young is formed in the egg. In such a case, the egg itself divides into
a number of portions: two, four, eight, or even twelve and sixteen
individuals being normally developed from every egg, in consequence of
this singular process of segmentation of the yolk,--which takes place,
indeed, in all eggs, but in those which produce but one individual is
only a stage in the natural growth of the yolk during its transformation
into a young embryo. As the facts here alluded to are not very familiar
even to professional naturalists, I may be permitted to describe them
more in detail.

No one who has often walked across a sand-beach in summer can have
failed to remark what the children call "sand saucers." The name is not
a bad one, with the exception that the saucer lacks a bottom; but the
form of these circular bands of sand is certainly very like a saucer
with the bottom knocked out. Hold one of them against the light and you
will see that it is composed of cou