Botanists are divided in opinion whether the innumerable varieties of manioc should be regarded as forming one, two, or several different species. Pohl[180] admitted several besides his _Manihot utilissima_, and Dr. Müller,[181] in his monograph on the Euphorbiaceæ, places the variety _aipi_ in an allied species, _M. palmata_, a plant cultivated with the others in Brazil, and of which the root is not poisonous. This last character is not so distinct as might be believed from certain books and even from the assertions of the natives. Dr. Sagot,[182] who has compared a dozen varieties of manioc cultivated at Cayenne, says expressly, “There are maniocs more poisonous than others, but I doubt whether any are entirely free from noxious principles.” It is possible to account for these singular differences of properties in very similar plants by the example of the potato. The _Manihot_ and _Solanum tuberosum_ both belong to suspected families (_Euphorbiaceæ_ and _Solanaceæ_). Several of their species are poisonous in some of their organs; but the fecula, wherever it is found, is never harmful, and the same holds good of the cellular tissue, freed from all deposit; that is to say, reduced to cellulose. In the preparation of cassava, or manioc flour, great care is taken to scrape the outer skin of the root, then to pound or crush the fleshy part so as to express the more or less poisonous juice, and finally the paste is submitted to a baking which expels the volatile parts.[183] Tapioca is the pure fecula without the mixture of the tissues which still exist in the cassava. In the potato the outer pellicle contracts noxious qualities when it is allowed to become green by exposure to the light, and it is well known that unripe or diseased tubers, containing too small a proportion of fecula with much sap, are not good to eat, and would cause positive harm to persons who consumed any quantity of them. All potatoes, and probably all maniocs, contain something harmful, which is observed even in the products of distillation, and which varies with several causes; but only matter foreign to the fecula should be mistrusted. The doubts about the number of species into which the cultivated manihots should be divided are no source of difficulty regarding the question of geographic origin. On the contrary, we shall see that they are an important means of proving an American origin. The Abbé Raynal had formerly spread the erroneous opinion that the manioc was imported into America from Africa. Robert Brown[184] denied this in 1818, but without giving reasons in support of his opinion; and Humboldt,[185] Moreau de Jonnes,[186] and Saint Hilaire[187] insisted upon its American origin. It can hardly be doubted for the following reasons:—