ctive ideas, which excite desire and arise through the influence
of the ganglionic system upon the brain; accordingly they are moved by a
certain illusion....

The great preponderance of brain in man accounts for his having fewer
instincts than the lower order of animals, and for even these few easily
being led astray. For instance, the sense of beauty which instinctively
guides a man in his selection of a mate is misguided when it degenerates
into the proneness to pederasty. Similarly, the blue-bottle (_Musca
vomitoria_), which instinctively ought to place its eggs in putrified
flesh, lays them in the blossom of the _Arum dracunculus_, because it is
misled by the decaying odour of this plant. That an absolutely generic
instinct is the foundation of all love of sex may be confirmed by a
closer analysis of the subject--an analysis which can hardly be avoided.

In the first place, a man in love is by nature inclined to be
inconstant, while a woman constant. A man's love perceptibly decreases
after a certain period; almost every other woman charms him more than
the one he already possesses; he longs for change: while a woman's love
increases from the very moment it is returned. This is because nature
aims at the preservation of the species, and consequently at as great an
increase in it as possible.... This is why a man is always desiring
other women, while a woman always clings to one man; for nature compels
her intuitively and unconsciously to take care of the supporter and
protector of the future offspring. For this reason conjugal fidelity is
artificial with the man but natural to a woman. Hence a woman's
infidelity, looked at objectively on account of the consequences, and
subjectively on account of its unnaturalness, is much more unpardonable
than a man's.

In order to be quite clear and perfectly convinced that the delight we
take in the other sex, however objective it may seem to be, is
nevertheless merely instinct disguised, in other words, the sense of the
species striving to preserve its type, it will be necessary to
investigate more closely the considerations which influence us in this,
and go into details, strange as it may seem for these details to figure
in a philosophical work. These considerations may be classed in the
following way:--

Those that immediately concern the type of the species, _id est_,
beauty; those that concern other physical qualities; and finally, those
that are merely relative and s