So that the numerous racial types now existing represent only such mutants as lie within the limits of tolerance imposed by the situation under which any given mutant type has emerged and survived. A surviving mutant type is necessarily suited more or less closely to the circumstances under which it emerged and first made good its survival, and it is presumably less suited to any other situation. With a change in the situation, therefore, such as may come with the migration of a given racial stock from one habitat to another, or with an equivalent shifting growth of culture or change of climate, the requirements of survival are likely to change. Indeed, so grave are the alterations that may in this way supervene in the current requirements for survival, that any given racial stock may dwindle and decay for no other reason than that the growth of its culture has come to subject the stock to methods of life widely different from those under which its type of man originated and made good its fitness to survive. So, in the mixture of races that make up the population of the Western nations a competitive struggle for survival has apparently always been going on among the several racial stocks that enter into the hybrid mass, with varying fortunes according as the shifting cultural demands and opportunities have favoured now one, now another type of man. These cultural conditions of survival in the racial struggle for existence have varied in the course of centuries, and with grave consequences for the life-history of the race and of its culture; and they are perhaps changing more substantially and rapidly in the immediate present than at any previous time within the historical period. So that, for instance, the continued biological success of any given one of these stocks in the European racial mixture has within a moderate period of time shifted from the ground of fighting capacity, and even in a measure from the ground of climatic fitness, to that of spiritual fitness to survive under the conditions imposed by a new cultural situation, by a scheme of institutions that is insensibly but incessantly changing as it runs.[9]