coordinate the metameres, or
primary agglutinated animals, into a single organism.

In the higher animals, the complex polycellular individuals formed by
the agglomeration of several primitive animals, are transformed into a
higher and mobile unity by the aid of the great vital apparatus called
the nervous system, which becomes the mental director of the living
organism and invests it with its individual character. However, this
higher unity of life, which always becomes more psychic, that is to
say, at the same time intellectual, sentimental and voluntary, by its
complication and its numerous relations with other individuals, this
unity called the _central nervous system_ cannot do without the
necessity for reproduction. In animal phylogeny, as soon as
hermaphrodism has ceased and each individual has become the sole
bearer of one of the two kinds of sexual cells, the species will
eventually disappear if the male cells cannot reach the female cells
by the active movement of the whole individual. Thus is produced the
marvelous phenomenon of the desire of increase and reproduction,
originally peculiar to the male cell, penetrating the nervous system,
that is to say life and soul in its entirety, the life of the higher
unity of the individual. An ardent desire, a powerful impulse thus
arises in the nervous system at the time of puberty and attracts the
individual toward the opposite sex. The care and the pleasure of self
preservation, which had hitherto fully occupied his attention, become
effaced by this new impulse. The desire to procreate dominates
everything. A single pleasure, a single desire, a single passion lays
hold of the organism and urges it toward the individual of the
opposite sex, and to become united with it in intimate contact and
penetration. It is as if the nervous system or the whole organism felt
as if it had for the moment become a germinal cell, so powerful is the
desire to unite with the other sex.

In some beautiful verses the German poet-philosopher _Goethe_
(West-Oestlicher Divan, book VIII, "Suleika") describes the desire to
procreate (p. 63):

Und mit eiligem Bestreben
Sucht sich, was sich angehoert,
Und zu ungemessnem Leben
Ist Gefuehl und Blick gekehrt.
Sei's ergreifen, sei es raffen,
Wenn es nur sich fasst und haelt!
Allah braucht nicht mehr zu schaffen,
Wir erschaffen seine Welt!

If we look at nature we see everywhere the same desire and the same
at