In the first cases mentioned above, in which the response appears to be of some advantage to the animal, the question may be asked, how have such responses arisen? The selection theory assumes that those animals that responded at first to a slight degree in a favorable direction have escaped, and this process being repeated, the power to change has been gradually built up. The mutation theory will also account for the result by assuming the response to have appeared as a new quality, but it has been preserved, not because it has been of vital importance to its possessor, but simply because the species possessing it has been able to survive, perhaps in some cases even more easily, although this is not essential. Even if the change were of no direct benefit, or even injurious to a slight degree, it might have been retained, as appears in fact to be the case in the change of color of the green lizards. Increase of Organs through Use and Decrease through Disuse We meet here with one of the most characteristic and unique features of living things as contrasted with non-living things. We shall have to dismiss at once the idea that we can explain this attribute of organisms by either the selection or the mutation theory; for we find animals possessing this power that could never be supposed to have acquired it by any experience to which they have been subjected; and since it appears to be so universally present, we cannot account for it as a chance mutation that may have appeared in each species. No doubt Wolff had responses of this kind in view when he made the rather sweeping statement that purposeful adaptation is the most characteristic feature of living things. The statement appears to contain a large amount of truth, if confined to the present group of phenomena. This power of self-regulation may confer a great benefit on its possessor. The increase in the size and strength of the muscles through use may give the animal just those qualities that make its existence easier. The increase in the power of vision, or at least of visual discrimination through use, of the power of smell and of taste, of hearing and of touch, are familiar examples of this phenomenon. However much we may be tempted to speculate as to how this property of the animal may have been acquired, we lack the evidence which would justify us in formulating even a working hypothesis. It may be that when we come to know more of what the process of contraction of the muscle involves, the possibility of its development as a consequence of its use may be found to be a very simple phenomenon that requires no special explanation at all to account for its existence in the individual, further than that the muscles are of such a kind that this is a necessary physical result of their action. But until we know more of the physiology involved in the process, it is idle to speculate about the origin of the phenomenon. Reactions of the Organism to Poisons, etc.