An individual is a separate detached being, and has nothing in common with other beings, excepting that it resembles, or rather differs from them. All similar individuals which exist on the earth, are considered as composing the species of those individuals. Notwithstanding, it is neither the number nor collection of similar individuals which form the species, but the constant succession and renewing of these individuals which constitute them; for, a being which existed for ever would not be a species. Species, then, is an abstract and general term, the meaning of which can only be determined on by considering nature in the succession of time, and in the constant destruction and renewal of beings. It is by comparing the present state of nature with that of the past, and actual individuals with former, that has given us a clear idea of what is called species: for a comparison of the number or resemblance of individuals, is only an accessory idea, and frequently independent of the first; for, the ass resembles the horse more than the barbet the greyhound, notwithstanding the latter are but one species, since they produce fertile individuals, but the horse and ass are certainly of different species, since they produce together vicious and unfruitful individuals. It is then in the characteristic diversity of the species, that the shades of nature are the most sensible and best marked; we may even say, that these shades between the species are the most equal and least variable, since we may always draw a line of separation between two species: that is, between two successions of individuals, who reproduce and cannot mix, as we may also unite into one species two successions of individuals which would reproduce by mixing. This is the most fixed point that we have in Natural History; all other resemblances, and differences that we can make in the comparison of beings, are neither so constant, real, nor certain. These intervals are the only lines of separation that will be found in this work; we shall not divide beings otherwise than they are in fact: each species, each succession of individuals which reproduce and cannot mix, will be considered apart, and treated separately; and we shall only make use of families, kinds, orders, and classes, which are marked out by Nature herself. Species, then, being nothing more than a constant succession of individuals alike, and which reproduce, ought only to extend to animals and vegetables, and that it is only an abuse of the term, and confounding ideas when used to point out the different kinds of minerals. We should not then look on iron as one species, and lead as another species, but only as two different metals, and should be distinguished by lines of separation different from those made use of with respect to animals and vegetables.