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 courtship.

The abhorrence of incest is an interesting and significant phenomenon from our present point of view, because it instructively points out to us the limits to that charm of parity which apparently makes itself felt to some considerable extent in the constitution of the sexual ideal and still more in the actual homogamy which seems to predominate over heterogamy.  This homogamy is, it will be observed, a racial homogamy; it relates to anthropological characters which mark stocks.  Even in this racial field, it is unnecessary to remark, the homogamy attained is not, and could not be, absolute; nor would it appear that such absolute racial homogamy is even desired.  A tall man who seeks a tall woman can seldom wish her to be as tall as himself; a dark man who seeks a dark woman, certainly will not be displeased at the inevitably greater or less degree of pigment which he finds in her eyes as compared to his own.