The opinion which in your letter of Jan. 24. you are pleased to ask of me on the comparative merits of the different methods of classification adopted by different writers on Natural history, is one which I could not have given satisfactorily even at the earlier period at which the subject was more familiar; still less after a life of continued occupation in civil concerns has so much withdrawn me from studies of that kind. I can therefore answer but in a very general way, and the text of this answer will be found in an observation in your letter, where speaking of nosological systems, you say that disease has been found to be an Unit. Nature has, in truth, produced Units only thro’ all her works. Classes, orders, genera, species are not of her work. her creation is of individuals. no two animals are exactly alike; no two plants, nor even two leaves or blades of grass; no two crystallisations. and, if we may venture, from what is within the cognisance of such organs as ours, to conclude on that beyond their powers, we must believe that no two particles of matter1 are of exact resemblance. this infinitude of Units, or individuals being far beyond the capacity of our memory, we are obliged, in aid of that, to distribute them into masses,2 throwing into each of these all the individuals which have a certain degree of resemblance; to subdivide these again into smaller groupes, according to certain points of dissimilitude observable in them; and so on, until we have formed what we call a system of classes, orders, genera, and species. in doing this we fix arbitrarily on such characteristic resemblances & differences as seem to us most prominent & invariable in the several subjects, and most likely to take a strong hold in our memories. thus Ray formed one classification on such lines of division as struck him most favorably; Klein adopted another; Brisson a third, and other naturalists other designations, till Linnaeus appeared. fortunately for science, he concieved in the three kingdoms of nature, modes of classification which obtained the approbation of the learned of all nations. his system was accordingly adopted by all, and united all in a general language. it offered the three great desiderata 1. of aiding the memory to retain a knolege of the productions of nature. 2. of rallying all to the same names for the same objects, so that they could communicate understandingly on them. and 3. of enabling them, when a subject was first presented, to trace it by it’s characters up to the conventional name by which it was agreed to be called. this classification was indeed liable to the imperfection of bringing into the same group individuals, which, tho resembling in the characteristics adopted by the author for his classification, yet have strong marks of dissimilitude in other respects. but to this objection every mode of classification must be liable, because the plan of creation is inscrutable to our limited faculties. Nature has not arranged her productions on a single & direct line. they branch at every step, and in every direction, and he who attempts to reduce them into departments is left to do it by the lines of his own fancy. the objection of bringing together what are disparata in nature lies against the classifications of Blumenbach & of Cuvier, as well as that of Linnaeus; and must for ever lie3 against all. perhaps not in equal degree; on this I do not pronounce. but neither is this so important a consideration as that of uniting all nations under one language, in Natural history. this had been happily effected by Linnaeus, and can scarcely be hoped for a second time. nothing indeed is so desparate as to make all mankind agree in giving up a language they possess for one which they have to learn. the attempt leads directly to the confusion of the tongues of Babel. disciples of Linnaeus, of Blumenbach, & of Cuvier, exclusively possessing their own nomenclatures, can no longer communicate intelligibly with one another. however much therefore we are indebted to both these naturalists, and to Cuvier especially, for the valuable additions they have made to the sciences of nature, I cannot say they have rendered her a service in this attempt to innovate in the settled nomenclature of her productions: on the contrary I think it will be a check on the progress of science, greater or less, in proportion as their schemes shall more or less prevail. they would have rendered greater service by holding fast to the system on which we had once all agreed, and by inserting into that such new genera, orders, or even classes as new discoveries should call for. their systems too, and especially that of Blumenbach, are liable to the objection of going too much into the province of anatomy. it may be said indeed that anatomy is a part of Natural history. in the broad sense of the word it certainly is. in that sense however it would comprehend all the natural sciences, every created thing being a subject of Natural history, in extenso. but in the subdivisions of general science, as has been observed in the particular one of Natural history, it has been necessary to draw arbitrary lines, in order to accomodate our limited views. according to these, as soon as the structure of any natural production is destroyed by art, it ceases to be a subject of Natural history, and enters into the domain ascribed to Chemistry, to Pharmacy, to anatomy Etc. Linnaeus’s method was liable to this objection so far as it required the aid of Anatomical dissection, as of the heart, for instance, to ascertain the place of any animal; or of a chemical process for that of a mineral substance. it would certainly be better to adopt as much as possible such exterior and visible characteristics as every traveller is competent to observe, to ascertain, and to relate. but with this objection, lying but in a small degree, Linnaeus’s method was recieved, understood, and conventionally settled among the learned, and was even getting into common use. to disturb it then was unfortunate. the new systems attempted in Botany by Jussieu, in Mineralogy by Haüy, are subjects of the same regret; and so also the no-system of Buffon; the great advocate of individualism, in opposition to classification. he would carry us back to the days, and to the confusion of Aristotle and Pliny, give up the improvements of twenty centuries, and co-operate with the Neologists in rendering the science of one generation useless to the next, by perpetual changes of it’s language. in Botany Wildenow and Persoon have incorporated into Linnaeus the new discovered plants. I do not know whether any one has rendered us the same service as to his Natural history. it would be a very acceptable one. the materials furnished by Humboldt, and those from New Holland particularly require to be digested into the Catholic system. among these, the Ornithorhyncus, mentioned by you, is an amusing example of the anomalies by which Nature sports with our schemes of classification. altho’ without Mammae, Naturalists are obliged to place it in the class of Mammiferae; and Blumenbach particularly, arranges it in his order of Palmipeds,4 and toothless genus, with the Walrus and Manatee. in Linnaeus’s system it might be inserted as a new genus, between the Anteater and Manis,5 in the order of Bruta. it seems in truth to have stronger relations with that class than any other, in the construction of the heart, it’s red and warm blood, hairy integuments, in being quadruped and viviparus, and, may we not say, in it’s tout ensemble, which Buffon makes his sole principle of arrangement? the Mandible, as you observe, would draw it towards the Birds, were not this characteristic overbalanced by the weightier ones beforementioned. that of the cloaca is equivocal, because, altho a character of birds, yet some mammalia, as the beaver and sloth have the rectum and urinary passage terminating at a common opening. it’s ribs also, by their number and structure are nearer those of the bird than of the Mammalia. it is possible that further opportunities of examination may discover the Mammae. those of the Opossum are asserted by the Chevalr d’Aboville, from his own observations on that animal made while here with the French army, to be not discoverable until pregnancy, and to disappear as soon as the young are weaned. the Duck-bill has many additional particularities which liken it to other genera, and some entirely peculiar. it’s description and history needs yet further information.

In what I have said on the Methods of classing I have not at all meant to insinuate that that of Linnaeus is intrinsically preferable to those of Blumenbach and Cuvier. I adhere to the Linnean because it is sufficient as a groundwork; admits of supplementary insertions, as new productions are discovered, and mainly because it has got into so general use that it will not be easy to displace it, and still less to find another which shall have the same singular fortune of obtaining the general consent. during the attempt we shall become unintelligible to one another, and science will be really retarded by efforts to advance it, made by it’s most favorite sons. I am not myself apt to be alarmed at innovations recommended by reason. that dread belongs to those whose interests or prejudices shrink from the advance of truth and science. my reluctance is to give up an universal language of which we are in possession without an assurance of general consent to recieve another. and the higher the character of the authors recommending it, and the more excellent what they offer, the greater the danger of producing schism.

I should seem to need apology for these long remarks to you who are so much more recent in these studies: but I find it in your particular request and my own respect for it, and with that be pleased to accept the assurance of my esteem and consideration.

Th: Jefferson