nesia the cocoanut was of the utmost importance, as many of the islands would have been uninhabitable without it. If its presence on these islands was due to cultivation, we have in it another important evidence of the colonisation of the region. Yam (Dioscorea alata).—The numerous species of the genus Dioscorea are scattered over the tropical portions of the Old and New Worlds. Many have large farinaceous rhizomes, which differ much in quality, some being good for human food, others having acrid or even poisonous properties. Yams, as these rhizomes are generally termed, have been used as food by the rude inhabitants of all tropical countries wherein they are found before they became acquainted with the art of agriculture. Baron von Mueller informs us that the aborigines of Australia consume large quantities of the roots of Dioscorea hastifolia, and that “it is the only plant on which they bestow any cultivation, crude as it is.”* “Select Extra-tropical Plants.” Baron F. von Mueller. Probably the yam was one of the first roots cultivated by man. In the New World several species were found in cultivation by the early European explorers; none of these exactly agreed with Old-World, species, but some of those found on the western side of the continent were allied to Japanese forms. The common yam (Dioscorea alata) was found in cultivation throughout Polynesia by the old European navigators. Another species, Dioscorea sativa, was also cultivated, but the rhizomes contained an acrid principle, and required a particular sort of cooking, hence it was less in vogue than D. alata, which seems to be foreign to the region, though its original habitat cannot be accurately determined, it being now very widely spread both on the mainland and the Asiatic islands. Since the European discovery of America the indigenous species cultivated there have been superseded by African and Asiatic species. This process of selection, when the cultivated plants of different regions become intermingled, is seen in the case of Triticum spelta, which is now only found in a few places in South Germany and Switzerland, having been driven out of cultivation by wheat (Triticum vulgare). As the spelt has not been discovered wild, it will probably become extinct should it cease to be cultivated. We may thus see that plants brought into cultivation at a very early period may have been subsequently lost through the invasion of their territory by species better fitted to supply the wants of the inhabitants. How readily a rude agricultural people adopt new plants that can be advantageously grown by them is seen, in Africa, where the manioc, maize, and sweet potato, all New-World species,