Purely instinctive acts, however, are probably best considered in animals too low in the scale of life to admit of our supposing that the adjustments which are produced could at any time have been intelligent; it being exceedingly difficult to measure the influence exerted by either, where instinct, necessity, or what Lamarck calls the law of inherited habit, becomes the partner of reason in the production of specific acts; and yet we should be but poorly guided, in considering the question of sexuality, did we limit our research to those animals destitute of that intelligence which constitutes the natural and frequently overlapping boundary of its domain.

An instinct comprises too many factors to be easily or accurately determined, however; and after we have carefully marshalled these factors in the mental field, according due consideration to each, and striving to separate them into such co-ordinated groups as would enable us to relegate each to a distinct order of activity, we are yet at a loss to dissociate the psychological from the physiological, the mental from the purely sensuous; and meet with possibly as many arguments in favor of the early view that the sexual impulse is simply one of glandular excretion, as of the later and more elaborately worked out opinion that there is a congenital instinct of procreation underlying the sexual need.