Adaptation of an organ in the opposite direction takes place when the function diminishes or ceases. If a nerve supplying a muscle or a gland be cut through, the organ concerned begins to degenerate and to lose its normal structure to a greater or less degree. Sensory nerves also degenerate in their peripheral part when they are cut through. In such cases there may be no alteration either in the nutritive mechanism or in the blood-vessels, &c., but the functional stimulus--in the case of the muscle, the stimulus from the will--no longer affects the organ, and its metabolism is so much lowered in consequence that it begins to degenerate. When we admit that the fit adaptation of the organism, as far as we understand it, must depend upon processes of selection, we may refer this 'functional adaptation' also to primitive processes of selection, which prevailed at the very beginning of life upon our earth, and represented, so to speak, the first adaptation that was established, but we can say nothing with certainty in regard to this matter as long as we do not understand the essence of assimilation. It is conceivable, however, that a _primary_ adaptiveness may have arisen, so to speak, abruptly, through a concurrence of favourable circumstances, as we shall endeavour to show later on when we discuss the beginnings of life. Even although we cannot lay bare the primary roots of 'functional adaptation' we can gain from the fact itself very valuable insight into phenomena which would otherwise be unintelligible and mysterious: _the perfectly adapted structure of many tissues and their power of adaptation to changed conditions_. In this lies, in the main, the advance in our knowledge which is due to Roux's _Kampf der Theile_.