Thus, in the most frequently quoted passage from his voluminous works, Buffon writes about the donkey:
 

This animal, when studied in great detail with attentive eyes, appears to be nothing but a degenerated horse.... One could attribute the slight differences between these two animals to the very ancient influence of climate, nutrition, and the fortuitous succession of many generations of small, partially degenerated wild horses. Little by little they would have degenerated so much that they would ultimately have produced a new and constant species.... Do the donkey and horse come originally from the same source? Are they, as the taxonomists say, of the same family? Or are they, and have they always been, different animals?
 
...(T)he body of the horse, which at first glance seems so different from that of man, nevertheless when compared in detail, part for part, the singular and most complete resemblance to be found is astonishing. Indeed, presented with the skeleton of man, you tilt the pelvic bone, shorten the femur, legs, and arms, elongate the feel and hands, fuse the phalanges, elongate the jaws by shortening the frontal bone, and finally elongate the spine and the skeleton will cease to represent the remains of a man and will be the skeleton of a horse....

If we trace in detail an essential aspect of the form, for example the ribs, they are found in man, in all the quadrupeds, in the birds, in the fish, and even as far as the turtle, where they seem to be laid out by the furrows beneath the shell. Consider, as M. Daubenton has said, that the foot of a horse, superficially so different from the hand of man, is nevertheless composed of the same bones; and that we have at the tips of each of our fingers the same horseshoe shaped bone which terminates the foot of the animal. It may be pondered whether this hidden resemblance is not more marvelous than the obvious differences and whether this constant conformity of design followed from man to quadrupeds, from quadrupeds to cetaceans, from cetaceans to birds, from birds to reptiles, from reptiles to fish, etc., seem not to indicate that in creating these animals, the supreme Being has wished to use an idea, and yet to vary it in all possible ways, so that man could admire equally the simplicity and magnificence of execution of this design.

From this point of view, not only the donkey and horse, but as well man, apes, quadrupeds, and all the animals could be regarded as constituting the same family. But must one conclude that within such a great and numerousfamily, which was called into existence from nothing by God alone, there were other smaller families, projected by nature and produced by time, some of which comprise but two individuals (like the horse and donkey),others of more individuals (like the weasel, ferret, martin, polecat, etc.) and likewise in the plants, that there were families of 10, 20, 30, etc. plants? Ifthese families indeed existed, they could only have been formed by the mixing, the successive variation and degeneration of original species. And ifit is once admitted that there are families of plants and animals, that the donkey is of the horse family, and that it differs only because it hasdegenerated, then one could equally say that man and ape have had a common origin like the horse and donkey that each family among the animals and vegetables have had but a single stem, and that all animals haveemerged from but a single animal which, through the succession of time, has produced by improvement and degeneration all the races of animals.

The naturalists who establish so casually the families of plants and animals do not seem to have grasped sufficiently the full scope of these consequences, which would reduce the immediate products of creation to a number of individuals as small as one might wish. For if it were once proved that these families could be established rationally that of the the animals and vegetables there were, I do not say several species, but only one, produced  by the degeneration of another species if it were true that the donkey were but a degenerated horse then there would be no limits to the power of nature. Once would then not be wrong to suppose that she could have drawn with time, all other organized beings from a single being.

But no: it is certain, from revelation, that all animals have participated equally in the grace of creation, that the pair of each species and of all species emerged fully formed from the hands of the Creator. And it must be thought that they were fairly close to their descendants, which now represent them. Moreover, since nature has been observed from the time of Aristotle, no new species has ever appeared, despite the speed by which particles of matter are broken down and dissipated, despite the infinite number of pairings which must have come about over 20 centuries, and despite the fortuitous or forced pairings of animals of different species, which always result in corrupted or sterile individuals, and which have not been able to found a new family over the generations. The internal and external resemblances, which are in some cases greater than the donkey and horse, must not lead us to confuse these animals, or to give them a common origin. For if they come from the same stem, if they are indeed the same family, it would be possible to bring them together, to cross them again, and undo with time what time has done.

...Thus, though it cannot be shown that the generation of new spoecies by degeneration exceeds the powers of nature, nevertheless the number of improbabilities involved makes it utterly unbelieveable. For if one species could be formed by the degeneration of another if the donkey really degenerated from the horse this change could only have come by a long succession of almost imperceptible degrees. Between the horse and donkey, there must have been many intermediate animals. The first of these would gradually retreat from the nature and characters of the horse, and the last  would make equal progress towards those of the donkey. What happened to these intermediate beings? Why are their representatives and descendants extinct? Why should the two ends alone exist?

Naturally Buffon's thought on evolution evolved. Thus, to represent the preceding excerpt from 1753 as expressing the totality of Buffon's views on the subject throughout his life would be misleading. Nevertheless this passage, actually an incomplete extract of it, formed the cornerstone of Samuel Butler's argument to support the view that Buffon was an evolutionist. If Butler seemed to be presenting black as white, he certainly recognized it as such, and did so by exceedingly clever argumentation. Butler argues that Buffon laughed up his sleeve while penning certain parts of the Natural History specifically, the less evolutionary parts. Fearful of further ecclesiastical censure, and yet prematurely evangelical for evolution, Buffon concealed his evolutionary sentiment with the camophlage of denial. Wrote Butler:
 

     I am inclined to think that a vein of irony pervades the whole, or much the greater part of Buffon's work, and that he intended to convey, one meaning to one set of readers, and another to another....
     What he did was to point so irresistibly in the right direction, that a reader of any intelligence should be in no doubt as to the road he ought to take, and then to contradict himself so flatly as to reassure those who would be so shocked by a truth for which they were not yet ready.
     It is hoped that the reader will see Buffon . . . as going into his library, etc., and quietly chuckling to himself as he wrote such a passage.

Reading the specific passage on the donkey should make it clear that Buffon's objections to trans specific evolution are numerous and profound, at least in the context of 18th century biology. More importantly, however, Buffon treated the question of evolution as a subset of a greater problem that of the reality of higher taxonomic categories, to which he is consistently and unequivocally opposed for the great bulk of his career.