As Professor Boas and American ethnologists generally have maintained,[10] many facts are quite inconsistent with the theory of unilinear evolution. That theory can be tested very simply by comparing the sequence of events in two or more areas in which independent development has taken place. For example, has technology in Africa followed the lines ascertained for ancient Europe? We know today that it has not. Though unlike southern Scandinavia, the Dark Continent is not lacking in copper deposits, the African Stone Age was not superseded by a Copper Age, but directly by a period of Iron. Similarly, I have already pointed out that the possession of the same domesticated animals does not produce the same economic utilization of them while the Tungus rides his reindeer, other Siberians harness their animals to a sledge; the Chinaman will not milk his cattle, while the Zulu’s diet consists largely of milk. That a particular innovation occurred at a given time and place is, of course, no less the result of definite causes than any other phenomenon of the universe. But often it seems to have been caused by an accidental complex of conditions rather than in accordance with some fixed principle. For example, the invention of the wheel revolutionized methods of transportation. Now, why did this idea develop in the Old World and never take root among the American Indians? We are here face to face with one of those ultimate data that must simply be accepted like the physicist’s fact that water expands in freezing while other substances contract. So far as we can see, the invention might have been made in America as well as not; and for all we know it would never have been made there until the end of time. This introduces a very important consideration. A given culture is, in a measure, at least, a unique phenomenon. In so far as this is true it must defy generalized treatment, and the explanation of a cultural phenomenon will consist in referring it back to the particular circumstances that preceded it. In other words, the explanation will consist in a recital of its past history; or, to put it negatively, it cannot involve the assumption of an organic law of cultural evolution that would necessarily produce the observed effect. Facts already cited in other connections may be quoted again by way of illustration. When a copper implement is fashioned not according to the requirements of the material, but in direct imitation of preëxisting stone patterns, we have an instance of cultural inertia: it is only the past history of technology that renders the phenomena conceivable. So the unwieldy Chukchee tent, which adheres to the style of a pre-nomadic existence, is explained as soon as the past history of the tribe comes to light.