The opinion that the genera of the same natural orders possessed similar medical powers, is by no means new. It was suggested in the Philosophical Transactions by Mr. Petiver, and is enforced by Hoffman in a separate dissertation. In 1784, a slight lively work appeared on this subject, entitled 'a Short Attempt to Recommend the Study of Botanical Analogy.' The author's aim seemed not to be deep investigation, but to catch the attention by a few prominent facts, for the purpose of a more complete examination of the natural orders by himself or others. No publication has, however, appeared in the same path, nor will the limits of the present article enable us to give more than an outline: we hope, however, to make it tolerably complete.

Several of the fragments of the natural orders left us by Linnaeus, contain no medical plant. They were published by the author without a comment; and we now find that he gave only at two very distant periods, lectures on this subject, viz. in 1764, to Ferber the future mineralogist, Fabricius the entomologist, to Zaega, to Kuhn of Philadelphia, and the chemist Meyer; and in 1771, to Giseke, Vahl, the celebrated botanist, to Edinger, and Tislef, From Giseke we have received an account of, these lectures, published at Hamburgh in 1792; a communication truly valuable, as he compared the manuscript of Fabricius with his own, and marks the agreement and differences.

The idea of natural orders includes the want of connection into classes and systems. Linnaeus, we shall find, had undoubtedly some views of connecting these fragments, but he often declared that he would not relate, on this subject, all that he knew. He was irritated at the injurious treatment different parts of his labours had received, and particularly the Fragments, so that he was with reluctance led to lecture on the subject. The formation of natural orders is a task of considerable labour, and we cannot better explain the difficulty than by translating the conversation between Giseke and Linnaeus: it is in many respects curious.

"Before, however, he began them (the lectures), we often disputed on the subject. I had thought that all the species of columniferae, for instance, should have a staminiferous column, and that those without it should be excluded from the order; while those distinguished by it, agreeing in other respects, thus formed the natural group. I thought the same of the bicornes, tri-coccae, and others. Linnaeus smiled, and told me, that we must not always determine from the name; that there were plants so nearly allied to the columniferae, though without that distinction, which could not be separated. He went on, and convinced me that there might be sometimes a single mark, which was found in all the plants of that order, which even gave it the name, but did not sufficiently distinguish it from others. Thus the siliquosae have each a siliqua, but no one in their senses will expect to find every siliquose plant in this order: the hypecoum, chelidonium, cleome, cap-paris, fumaria, and epimedium, are all siliquose; but how different! how irreconcilable! Hence, he added, those who, on account of the siliqua, unite them to the cruciformes, or, in consequence of this distinction, unite other orders with it, do not understand my orders. I replied, what then is the use of a name, if the mark pointed out by the appellation docs not suit every individual? It is of little consequence, he added, what the name is, so that it has some connection with the series of plants, and it is ascertained that you speak of a particular group which you have united. I have followed the axiom of the logicians, a potiori denominari. Can you give me the character of one order? G. I think I can; of the umbellatae, for instance.

"L. What is the character of the umbellatae ? G. That circumstance, viz. their flowers being disposed in an umbella. L. Right: but are there not plants whose flowers are umbellated that do not belong to it ? G. I recollect; and must therefore add two naked seeds. L. Then the echinophora will not be of this order, which bears its seed in the centre of the peduncle, yet it is umbellated: and to what order would you refer the eryngium? G, To the aggregatae. L. By no means: it is certainly umbellated; for it has an invo-lucrum, five stamina, and two pistils: what then is its character? G. Such plants should be placed at the end of the order, to connect it with other orders: perhaps the eryngium would unite the umbellatae with the aggregatae. L. It is a very different affair from giving the characters of orders to point out transitions. I know the orders, and their connections, but these I will not ex-plain: I will never explain. If, however, the character is a mark to distinguish the object from every other; if the orders are to be discriminated; if the agreement of the orders forms a class; and the connection of classes a method; we cannot have a natural method in botany, for we must first complete the characters of the orders: but this is impossible. Take any order you please, and you will see it cannot be done.

"G. The contortae have a very distinct character, viz. the contortion of the corolla, previous to its expansion. L. The malvaceae, however, have the same peculiarity of the leaves, though wholly different.

"G. Perhaps, then, the tricocca? L. Ah! thecambo-gia has an apple. G. And the hura a multivalvc capsule, the hippomane a fruit; so that they must be removed from the order. L. No, they cannot, and should not be removed: the connection is so intimate, and the other species of hippomanes, except the h. mancinella, are really tricoccae. G. I wonder, therefore, that you did not insert the tropeolus. L. (smiling), I wish you understood affinities ! In that case I should have opposed nature; and the rhamnus should be inserted also. G. Why not;? L. The celastrus cannot be separated from theeuonymus, which has five berries, nor this from the rhamnus. G. These things appear to me very obscure. L. I think so. There was, in 1771, one Fragaeus at Petersburgh, an assiduous man, who determined to find out a clavis for my orders, and, after labouring for nearly three years, sentme his ' scheme.' I laughed heartily, and replied, learn first what is a natural order, and I showed him. But this I know, if I publish another edition of the genera, I will make a new arrangement of the orders, and change them in many respects; for the former orders of the list are more nearly connected than the latter. I would also arrange the rest, so as to divide them into groups, equally natural with the first and second. G. Excluding, however, the ferns, and those which follow. L. Entirely; for these would make another group.