As regards mental constitution, although we cannot directly observe and measure these disharmonies of composition, there seems good reason to believe that they exist. The soul of the cross-bred is, it would seem, apt to be the scene of perpetual conflict of inharmonious tendencies. This has been the theme of many stories, and, though no doubt many of them are overdrawn, there is no reason to doubt that they in the main depict actual experience or are founded on close observation. It is on the moral, rather than the intellectual, side of the mind that the disharmony seems to make itself felt most strongly; and the moral detachment of the cross-bred from the moral traditions of both the parent stocks is possibly due in part to a certain lack of innate compatibility with those traditions, as well as to social ostracism; the cross-bred can assimilate neither tradition so easily and completely as the pure-bred stocks. It is possible, though this is a still more speculative view, that the same is true of the intellectual constitution of the mind. The superiority of subraces formed by the blending of allied stocks seems to fall principally under two heads: (1) a general vigour of constitution; (2) a greater variety and variability of innate mental qualities. The greater variability of qualities of a subrace renders that race more adaptable to changing conditions; for racial adaptability depends upon the occurrence of abundant spontaneous variations. A large variety of innate qualities renders a race capable of progressing rapidly in civilisation; it renders it more capable both of producing novel ideas and of appreciating and assimilating the ideas, discoveries, and institutions of other peoples; and such imitative assimilation from one people to another has been a main condition of the progress of culture. It is, of course, well recognised that the great centres of development of culture have been the places where different peoples have come most freely into contact, notably the centre of the old world where Asia, Africa, and Europe meet together. This was the area in which the three great races of Europe came first into contact and mingled freely. Some authors attribute the fertilising influence upon culture wholly to the blending or contact of cultures; but there is good reason to believe that it is largely due also to race-blending.