In the case of many of the phenomena which may be explained from these points of view, it is quite impossible to give incontrovertible arguments which would prove that these customs are not due to parallel and independent development rather than to community of origin: the decision of this problem will be found largely in the results of prehistoric archæology on the one hand, and in those of animal psychology on the other. The problem is made still more difficult by the dissemination of cultural elements from tribe to tribe, from people to people, and from continent to continent, which can be proved to have existed from the earliest times on. As an instance of the rapidity with which cultural achievements are transmitted may be mentioned the modern history of some cultivated plants. Tobacco and cassava were introduced into Africa after the discovery of America, and it took little time for these plants to spread over the whole continent; so that at present they enter so deeply into the whole culture of the negro, that nobody would suspect their foreign origin (Hahn). We find in the same way that the use of the banana has pervaded almost the whole of South America (Von den Steinen); and the history of Indian-corn is another example of the incredible rapidity with which a useful cultural acquisition may spread over the whole world. It is mentioned as known in Europe in 1539, and, according to Dr. Laufer, had reached China by way of Tibet between 1540 and 1570. It is easy to show that similar conditions prevailed in earlier times. Victor Hehn’s investigations show the gradual and continuous increase of the number of domesticated animals and cultivated plants, due to their importation from Asia. The same process was going on in prehistoric times. The gradual spread of the Asiatic horse, which was first used as a draught animal, later on for riding, the spread of cattle over Africa and Europe, the development of European grains, may serve as illustrations. The area over which these additions to the stock of human culture were spread is very large. We see most of them travel westward until they reach the Atlantic coast, and eastward to the shores of the Pacific Ocean. They also penetrated the African Continent. It may be that the use of milk was disseminated in a similar way; for when the people of the world enter into our historic knowledge, we find milk used all over Europe, Africa, and the western part of Asia.