The discovery of America in 1492 was the last great event which caused the diffusion of cultivated plants into all countries. The American species, such as the potato, maize, the prickly pear, tobacco, etc., were first imported into Europe and Asia. Then a number of species from the old world were introduced into America. The voyage of Magellan (1520-1521) was the first direct communication between South America and Asia. In the same century the slave trade multiplied communications between Africa and America. Lastly, the discovery of the Pacific Islands in the eighteenth century, and the growing facility of the means of communication, combined with a general idea of improvement, produced that more general dispersion of useful plants of which we are witnesses at the present day. 5. _Philology._ The common names of cultivated plants are usually well known, and may afford indications touching the history of a species, but there are examples in which they are absurd, based upon errors, or vague and doubtful, and this involves a certain caution in their use. I could quote a number of such names in all languages; it is enough to mention, in French, _blé de Turquie_, maize, a plant which is not a wheat, and which comes from America; in English, Jerusalem artichoke (_Helianthus tuberosus_), which does not come from Jerusalem, but from North America, and is no artichoke. A number of names given to foreign plants by Europeans when they are settled in the colonies, express false or insignificant analogies. For instance, the _New Zealand flax_ resembles the true flax as little as possible; it is merely that a textile substance is obtained from its leaves. The _mahogany apple_ (cashew) of the French West India Isles is not an apple, nor even the fruit of a pomaceous tree, and has nothing to do with mahogany. Sometimes the common names have changed, in passing from one language to another, in such a manner as to give a false or absurd meaning. Thus the tree of Judea of the French (_Cercis Siliquastrum_) has become the Judas tree in English. The fruit called by the Mexicans _ahuaca_, is become the _avocat_ (lawyer) of the French colonists. Not unfrequently names of plants have been taken by the same people at successive epochs or in different provinces, sometimes as generic, sometimes as specific names. The French word _blé_, for instance, may mean several species of the genus Triticum, and even of very different nutritious plants (maize and wheat), or a given species of wheat. Several common names have been transferred from one plant to another through error or ignorance. Thus the confusion made by early travellers between the sweet potato (_Convolvulus Batatas_) and the potato (_Solanum tuberosum_) has caused the latter to be called potato in English and _patatas_ in Spanish.