And Classis, (from classes facere, and ultimately from to divide). Classification may perhaps scarcely at first appear to be a subject which belongs to the present work; but as we wish not to conceal that we consider the arrangement of diseases as an object of importance, and as we have tacitly acquiesced in the propriety of the classification of plants, animals, and minerals, connected with medicine, by adopting the plans of naturalists, it is proper in this place to explain their principles. .

Nature, it is said, has created only species: it is not true; for she has created only individuals. The similarity of these has occasioned the establishment of spe-cies; for similar individuals form a species. Individuals, differing in circumstances arising from accident; in plants and animals, from soil and climate; in diseases, from constitution; in minerals, from locality, are styled varieties: and these, when circumstances are changed, return to the species from which they started. These distinctions, though apparently simple and obvious, are, however, necessary;• for naturalists have usually begun at the other extremity, and formed "methods"(see Botany), classes and orders, before they have established species, and, at this moment, in nosology and mineralogy, the great impediments to improvement arise from the uncertainty of what are species. Even in botany this difficulty was once so great, that more than half of Tournefort's supposed species have been found to be varieties only. Three fourths of Sauvages' species of diseases are varieties or symptoms. This latter subject we hope in future to illustrate. (See Nosology). Having shortly then pointed out the distinctions between species and varieties, as well as the means by which the former are ascertained, we shall next consider genera. This is the first step in arrangement; for the establishment of species consists in ascertaining identity; of genera, similarity. A striking discriminating mark, in many species, sometimes establishes a genus; at others, a general similarity. The conduct of botanists, however, has differed in this part of their labour, from the difference of their dispositions. Some naturalists, catching hastily at analogies, have included numerous species under a genus: others, more wary and exact, have retrenched them too rigorously. The latest botanists have rendered the genera more, sometimes too, numerous; but this of the two is the more venial error, since new discoveries continually enlarge them.

An order is an association of genera; but orders are usually too comprehensive, including too great a number of genera; and, to facilitate investigation, these are often divided into separate groups, as in mineralogy the species are sometimes again divided into sub-species. Each is a proof of imperfection in arrangement.

A class contains the different orders; and though, in reality, it should be the last, or nearly the last, labour, it has usually been the first; and, to make the system elegant in appearance, the classes have been few and comprehensive. The classes are connected by what in botany is styled a "method"which we have already mentioned. Thus, in the Linnaean system of plants, they are said to have evident or concealed fructification; and in nosology Dr. Cullen has first divided diseases into general and local, forgetting that with little change of appearance or treatment they pass insensibly into each other.