Aland's question why the ten commandments should not now be a part of the common law of England? we may say they are not because they never were made so by legislative authority, the document which has imposed that doubt on him being a manifest forgery. TO DR. JOHN MANNERS. MONTICELLO, February 22, 1814. 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at which the subject was more familiar; still less, after a life of continued occupation in civil concerns has so much withdrawn me from studies of that kind.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.