rth in the previous volume of these _Studies_
will quickly perceive that the normal failure of the pairing instinct to
manifest itself in the case of brothers and sisters, or of boys and girls
brought up together from infancy, is a merely negative phenomenon due to
the inevitable absence under those circumstances of the conditions which
evoke the pairing impulse. Courtship is the process by which powerful
sensory stimuli proceeding from a person of the opposite sex gradually
produce the physiological state of tumescence, with its psychic
concomitant of love and desire, more or less necessary for mating to be
effected. But between those who have been brought up together from
childhood all the sensory stimuli of vision, hearing, and touch have been
dulled by use, trained to the calm level of affection, and deprived of
their potency to arouse the erethistic excitement which produces sexual
tumescence.[187] Brothers and sisters in relation to each other have at
puberty already reached that state to which old married couples by the
exhaustion of youthful passion and the slow usage of daily life gradually
approximate. Passion between brother and sister is, indeed, by no means so
rare as is sometimes supposed, and it may be very strong, but it is
usually aroused by the aid of those conditions which are normally required
for the appearance of passion, more especially by the unfamiliarity caused
by a long separation. In reality, therefore, the usual absence of sexual
attraction between brothers and sisters requires no special explanation;
it is merely due to the normal absence under these circumstances of the
conditions that tend to produce sexual tumescence and the play of those
sensory allurements which lead to sexual selection.[188] It is a purely
negative phenomenon and it is quite unnecessary, even if it were
legitimate, to invoke any instinct for its explanation. It is probable
that the same tendency also operates among animals to some extent, tending
to produce a stronger sexual attraction toward those of their species to
whom they have not become habituated.[189] In animals, and in man also
when living under primitive conditions, sexual attraction is not a
constant phenomenon[190]; it is an occasional manifestation only called
out by the powerful stimulation. It is not its absence which we need to
explain; it is its presence which needs explanation, and such an
explanation we find in the analysis of the phenomena of courtsh