After saying that "_la marche de la Nature_" has been misunderstood, and that her progress has ever been by a succession of slow steps, he maintains that the only proper course is to class together whatever objects resemble one another, and to separate those which are unlike. If individual specimens are absolutely alike, or differ so little that the differences can hardly be perceived, they must be classed as of the same species; if the differences begin to be perceptible, but if at the same time there is more resemblance than difference, the individuals presenting these features should be classed as of a different species, but as of the same genus; if the differences are still more marked, but nevertheless do not exceed the resemblances, then they must be taken as not only specific but generic, though as not sufficient to warrant the individuals in which they appear, being placed in different classes. If they are still greater, then the individuals are not even of the same class; but it should be always understood that the resemblances and differences are to be considered in reference to the entirety of the plant or animal, and not in reference to any particular part only.[68] The two rocks which are equally to be avoided are, on the one hand, absence of method, and, on the other, a tendency to over-systematize.[69] Like Dr. Erasmus Darwin, and more recently Mr. Francis Darwin, Buffon is more struck with the resemblances than with the differences between animals and plants, but he supposes the vegetable kingdom to be a continuation of the animal, extending lower down the scale, instead of holding as Dr. Darwin did, that animals and vegetables have been contemporaneous in their degeneration from a common stock. "We see," he writes, "that there is no absolute and essential difference between animals and vegetables, but that Nature descends by subtle gradations from what we deem the most perfect animal to one which is less so, and again from this to the vegetable. The fresh-water polypus may perhaps be considered as the lowest animal, and as at the same time the highest plant."[70] Looking to the resemblances between animals and plants, he declares that their modes of reproduction and growth involve such close analogy that no difference of an essential nature can be admitted between them.[71]