But these judicious and stimulating, if slightly vague, appeals for the conversion of natural history into a science of causal relations and generalized laws, were not the principal purpose of the preliminary discourse. The thought of Buffon at the time when he wrote that essay seems to have been dominated above all by a single idea, which was also one of the two or three ruling ideas of the whole of the first half of the eighteenth century—namely, the Leibnitian "principle of continuity'* (lex continui). In the intellectual fashions of this period, next to the blessed word "Nature" the most sacred phrase was "the Great Chain of Beings"; indeed, one of the truths that man was supposed to know most surely about nature was that she "makes no leaps." In the form, especially, of the neo-Platonic and Spinozistic metaphysical assumption that all possible forms must exist, the principle was much older than the philosophy of Leibniz;[1] but it owed to him and his disciples a more definite formulation and a greatly increased popular currency. It declared that all entities are arranged in a graded scale of similarity, so that for every being that exists there also exists some other (in the strict version of the principle, one and only one other) from which its difference is infinitesimal, i. e., less than any assignable difference. A typical statement of the doctrine is Bonnet's:[2]
Between the lowest and the highest degree of corporeal or spiritual perfection there is an almost infinite number of intermediate degrees. The series of these degrees constitutes the Universal Chain. It unites all beings, binds together all worlds, embraces all spheres. One Being alone is outside of this chain, and that is He who made it. . . . There are no breaks (sauts) in nature; all is graduated, everything shades off into the next thing. If, between any two beings whatever, there existed a gap, what would be the reason of the transition from the one to the other? There is, therefore, no being above or below which there is not some other that approximates it with respect to some characters and diverges from it with respect to others.
All this (as Bonnet's language intimates) was held by the Leibnitian philosophy to be logically implied by the still more fundamental "principle of sufficient reason." For if the gradations found in nature
