You have given me great pleasure by sending me your excellent volume on geology. it might otherwise have been long unknown to me; as we are late here even in hearing of the new publications of Europe. altho’ my engagements leave me time to read scarcely any thing, yet I could not resist the subjects of this volume. I had paid considerable attention some five & twenty years ago to the fossile bones on the Ohio. at that time that country was little explored, much less was known of the bones than at present, & they had been less the subject of discussion with the literati of Europe. M. de Buffon had given a first opinion that they were not the bones of an Elephant, in which Hunter had concurred. but M. de Buffon afterwards changed his opinion, and with this latter opinion Daubenton concurred. We had, for the ground of our judgment here, only the grinders, a few bones, & the climate they were found in. the grinders were marked by strong differences from those of the elephant: the bones were of vastly greater volume: the climate too rigorous for the elephant we knew. these differences were thought sufficient to deem this animal of a different genus. yet as genera & species are mere matters of definition, depending on the characteristics which each author of a system assumes as his rule of distribution, there can be no real objection to assigning to the American animal a chamber in the apartment of elephants, and to the Mammoth another and a distinct one, it being understood that ulterior comparisons shew them to be distinct species. your distribution of these animals therefore into 4. species, the Asiatic, the African, the Mammoth, and the American, is well calculated to reconcile opinions, & has just claims to adoption. the objections derived from climate may be solved, either, 1. by supposing that some great convulsion of our globe, some displacement of the ocean perhaps, has transported these remains from the places of original deposit to those where they are now found: altho’ it would perhaps be remarked as singular that the same convulsion had brought none of the bones of the Asiatic or African elephant here, nor left any of those of the Mammoth, or American, there. or 2. by supposing that nature may have distinguished the different species of elephants by differences of temperament, as we see done in the case of the brown bear, & the white sea-bear: and in that of the round-horned elk, of America, & the palmated elk (called Moose in America) the former of which does not exist here North of 42.° nor the latter South of that line. between these two solutions every one will be able to satisfy himself with the classification you have so judiciously proposed.

The remains of the Megalonyx were discovered here in 1796. we had then never heard of the animal of Paraguay, whose skeleton is mounted at Madrid. we had only the foreleg compleat, & the claws, of ours, to guide us. we naturally therefore brought him into comparison with the clawed animals already known; and, of these, with the lion as the largest. it was not till 1797. that we obtained some knolege of the description by messrs. Cuvier & Roumé of the animal of Paraguay, & that through the imperfect channel & coarse engraving of an English magazine. these gentlemen having the whole skeleton before them, and particularly the teeth, as well as the claws, had found that, according to those methods of arrangement which assort quadrupeds by their teeth & claws, this one would take his station with the Sloth. nature deals only in individuals. classification is the mere work of man, to help his research and recollection, and different zoologists have assumed different characteristics for classification. hence it has happened that all of these methods sometimes associate species the most unlike in their general characters. the present instance is remarkable, as it brings together what nature has placed in her extremes, that is, the Bradypus, the slowest & most helpless of all animals, with the Megalonyx, in all probability the most powerful which has existed. in an arrangement which, like M. de Buffon’s, presents animals in groupes, according to their general and most obvious analogies, I presume the Megalonyx would take his stand, where we had reviewed him, at the head of the Unguiculated column. still I think with you he had better be kept in reserve, as an animal sui generis, until further information may enable us to place him among companions with whom he will appear not dissorted.

A journey of discovery undertaken here will probably produce some further information as to the Megalonyx & other animals, lost as well as living. it’s immediate object is to explore the Missouri river to it’s source, proceed thence to the nearest river leading Westward, & descend that to the Pacific ocean; so as to give us with accuracy the geography of this interesting channel of communication across our continent. the labour & perils of this journey, the athletic constitution it required, hardy habits, knolege of Indian manners, and address in managing them, excluded from the enterprise men merely scientific, whose habits render them generally unequal to such an undertaking. Capt. Lewis, to whom I confided it, is perfectly equal to the Astronomical duties: and tho’ neither a regular botanist, zoologist, or mineralogist, has been so accurate an observer of all the subjects of these departments existing here, that he will not lose his time in noting things already known. he will select such only as are new to this part of the world. he will particularly give a good account of new animals. this party, consisting of about 12. persons, will probably return at the close of 1805.

I am in hopes of being able, the ensuing summer, to send other parties up the principal branches of the Missisipi and Missouri; the Red river, Arcansa, the Padoucas, Panis, & the Missisipi itself. the objects will be the same with those confided to Capt. Lewis. these also will require two years to accomplish their mission, some of these rivers extending 1000. or 1200. miles from their mouths, and into a country as yet unexplored by whites. it will give me great pleasure if these enterprises should produce materials for extending the bounds of knolege, and enable us to yield to our elder brethren in science some return for the accumulated stores of twenty centuries which we have recieved from them.

I pray you to accept my salutations and assurances of great respect and consideration.