SIR,--The opinion which, in your letter of January 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at which the subject was more familiar; still less, after a life of continued occupation in civil concerns has so much withdrawn me from studies of that kind.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 through 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 crystallizations. And if we may venture from what is within the cognizance of such organs as ours, to conclude on that beyond their powers, we must believe that no two particles of matter 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, throwing into each of these all the individuals which have a certain degree of resemblance; to subdivide these again into smaller groups, 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 and differences as seem to us most prominent and 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 Linnæus appeared. Fortunately for science, he conceived 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: First, of aiding the memory to retain a knowledge of the productions of nature. Secondly, of rallying all to the same names for the same objects, so that they could communicate understandingly on them. And Thirdly, of enabling them, when a subject was first presented, to trace it by its character 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, though 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 and 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 and of Cuvier, as well as that of Linnæus, and must forever lie 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 Linnæus, and can scarcely be hoped for a second time. Nothing indeed is so desperate 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 Linnæus, of Blumenbach, and 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.