Nevertheless, an objection may plausibly be raised here. Though an explanation has certainly been given, it does not account for all aspects of the phenomena we are considering. There is a psychological basis for each and every one of the events in our historical series. This series we may subdivide into three stages--the invention by the Chinese, the borrowing of this invention by the Arabs, and its transmission from Arab to European. Now the two last-named processes of transmission may not suggest the necessity of a special explanation at all. One may think that all that was required was for the Europeans to watch the Arabs and for the Arabs to watch the Chinese, and presto! the thing was done. This indeed, seems to be the view of an influential school of modern ethnologists. But the case is far from being so simple. We know of many instances, in the higher no less than in the lower cultures, corresponding to what the biologist calls symbiosis--a condition where distinct communities or countries persist in a division of labor for mutual benefit, each trading some of its intellectual or material products for equivalents secured from the other. In many parts of Africa there are fixed markets in which negroes from fairly remote localities congregate for the barter of wares, which are thus diffused far from their source of origin; but it is the finished products, not the arts, that are diffused. In New Guinea trading-vessels carry such objects as pottery hundreds of miles from the area of manufacture to natives who remain as ignorant of the ceramic technique as before. In northern Arizona the Hopi Indians occupying three eminences not more than eight miles distant from one another have no perfect uniformity of industrial knowledge. Pottery, which flourishes on the eastern Mesa, is wholly unknown as an art, though constantly used in its specimens, by the people of the central Mesa; a certain type of basketry plaque is made only at Oraibi village; another type is manufactured exclusively on the central Mesa. Conditions more ideal _a priori_ for a transfer of knowledge than among the practically homogeneous neighboring Hopi groups could not be conceived. Nevertheless, it has not taken place. Cultural diffusion, therefore, cannot be taken for granted. We cannot take one people, place it alongside of another, and effect a cultural osmosis in the same way in which we produce a chemical reaction when two substances are brought together under proper conditions of temperature. We are face to face with a selective, with a _psychological_ condition. But when we turn once more to our text-books of psychology, we again find nothing that fits the case. About choice in general we get ample information. But we may rummage all the psychological seminar rooms in the world and yet shall find no reason why the Arabs learned the technique of paper-making from the Chinese instead of ignoring it or only importing Chinese paper.