The complexity of this response stands in very close relation to the structural complexity of the organism in which it occurs, and this because an ever-increasing number of mechanisms and actions must be set in motion to carry out the fulfilment of any given stimulus, as this is traced from the lower to the higher groups of animals: till at last we have to distinguish between movements that are merely reflexes, and those which are “instinctive.” The latter must be fulfilled by the former—the reflex actions are the agents of the instinctive. Indifferent performance in either, endangers the existence of the individual, and in some directions of the race itself. The sexual instincts, with which alone these pages are concerned, are primarily stimulated and sustained by internal forces, generated, as we have already seen, by the juices of certain glands whose relation to the reproductive system has only recently been discovered. Though not commonly realized, and though denied by some, the sexual instincts are the dominant factors in the animal world. Even Man himself, the lord of Creation, knowing good and evil, cannot escape their overmastering rule. Commonly he is by no means inclined to rebel against this control But there be some who, in their arrogance, imagine that its overthrow is an end to be desired. Having scaled some slight intellectual eminence they fondly imagine this feat was accomplished by virtue of some spiritual grace of their own cultivation, and call to their fellow-men to emulate their example. But such preceptors are labouring under a strange delusion: they are suffering from a disease they wot not of, a “Disharmony,” as Metschnikoff calls it, a disease which blinds their perception of the motive power which has given them all that they believe themselves to have created. For these same despised instincts are the sacred fires of our being, and when they are quenched all that makes us human, love, ambition, and life itself will be extinguished. If the continuance of the race be a thing to be desired it is well that the choice should not be left to us. Truisms are sometimes trite, and while it is a truism to say that no race can continue which does not reproduce its kind, it is more exact to say that, other things being equal, the race depends for its existence, primarily, on the efficient working of the sexual instincts. In the higher animals, the phenomena which these present are so complex that they often assume something more than a semblance of intelligent, purposeful behaviour. It is therefore necessary, for their right understanding, that they should be analysed in animals lower and lower in the scale of life till at last we come to the very simplest types of organisms wherein instinct can be said to play a part.