Imperfect as is our present knowledge of the manner in which the nervous connections implied in psychological associations are established, there can be no question that they are acquired in the course of individual life; they are modifications of nervous structure due to a special mode of use under the conditions of experience. Here, then, in the case of the nervous system, as in that of the bodily organs before mentioned, two co-operating factors determine the limits of efficiency--heredity and use. Just as the heart and lungs must inherit the power of standing abnormal strain if the animal is to avoid elimination in times of unwonted exertion, so must the nervous system inherit some reserve power of dealing effectively with unwonted circumstances by intelligent accommodation, if the animal is not to fall a victim to such circumstances. In other words, at times of heightened competition those animals which can draw on a reserve fund of intelligent accommodation will survive, while the stupid blunderers will be eliminated. We may term this reserve fund of intelligent accommodation, this inherited ability to meet specially difficult circumstances as they arise, _innate capacity_. From the nature of the case it must be indefinite, for it must carry with it the ability to meet unforeseen combinations of the environing forces by new combinations of the results of experience. Its distinguishing mark is plasticity, in contradistinction to the stereotyped fixity of typical instinct. And accompanying its evolution there is probably, as we have seen, a dissolution of its antithesis, instinct. Thus may we account for the fact that man, with his great store of innate capacity, has so small a number of stereotyped instincts. But the dissolution of instincts is not complete. Residua are left in the inherited mental constitution. And these we term _congenital tendencies_ and _propensities_. They differ from the typical instincts in the fact that the definiteness of response has been lost. They dictate a general trend of action, but the particular application in behaviour is due to intelligent accommodation. They are commonly spoken of as instinctive; and their mode of origin justifies the use of the adjective in association with the term “propensities.” But it must be remembered that the behaviour to which they lead is not, as such, wholly instinctive; it is a joint product of instinct and intelligence, the general trend being due to the instinctive propensity, while the mode of application is guided by intelligence.