At the outset there is enormous waste incurred in the liberation of sperms and ova into the water, there to unite as best they may. Congress of the sexes eliminates the major part of this waste, and is universal above a certain level. This is in itself the basis for other changes. As the mind, or shall we say the psycho-neural organization, becomes more complex, the sexual instinct becomes more interwoven with the general emotional state; and a large number of animals appear not to mate unless their emotional state has been raised to a certain level. The result of this is that special actions, associated generally with bright colours or striking structures, with song or with scent, come into being. The exact mechanism of the appearance of these courtship-displays is a much-vexed point; but it is undoubted that they only occur in animals with congress of the sexes and with minds above a certain level of complexity, and that they are employed in ceremonies between the two sexes at mating-time. There can subsist no reasonable doubt that there exists some causal connection between the associated facts. An important point, which has been commonly overlooked, is that such characters and actions may be either developed in one sex only, or in both. In a large number of birds, such as egrets, grebes, cranes, and many others, the courtship-displays are mutual, and the characters used in them developed to a similar extent in both sexes. Such characters are therefore often not secondary sexual differences, and we had best use Poulton’s term _epigamic_ for them, whether they are developed in one or in both sexes.[29] The human species, in accordance with its complexity and flexibility of brain, has epigamic characters of both kinds. Some, like voice and moustache, are different in the two sexes, others, such as colour of eyes and lips, the hairlessness of the body and grace of limbs and carriage, are common to both. In the vertebrate stock, two main lines of evolution as regards sexual relationships may be traced. The first is predominant in mammals: here, in most species, the female will not receive the male except at fixed times, which are determined by a purely physiological mechanism, the internal secretion of the gonad (reproductive organ). Here we consequently find that the rule is for the males to fight for the possession of the females, not to display before them. In the monkeys, presumably as a result of a lessened dependence of mental upon physiological processes, bright colours and special adornments of various parts of the body are frequently developed.[30]