I will cite from Barr's Buffon (Buffon's Natural History Containing a Theory of the Earth, a general history of man, of the brute creation, and of vegetables, minerals, &c. From the French. With notes by the translator. London: J. S. Barr) published in ten volumes between 1792 and 1807.
Although Buffon rejected the idea that species could be transformed into new species, he discussed it in terms that set the discourse for later naturalists. Buffon rejected the idea, because he believed that species had been created in a perfect state and that varieties and races were degenerations from this perfect state. A transformation of on species into another was therefore a contradiction in his terms. New species were perfect, while transformations were degenerative. How could a process of degeneration lead to a new, that is, perfect species? Later naturalists only had to change his terms and to replace, in particular, the idea that variation was necessarily degenerative.
Here's a relevant passage form page 192f of volume 5 in Barr's Buffon (see here or here):
"But what an immense number of combinations are even necessary, even to suppose that two animals, male and female, of a certain species, have so much degenerated as to form a new species, and are no longer able to produce with any of their own kind but themselves! And also to suppose that the procreation of these two degenerated animals should follow exactly the same laws which are observed in the procreation of perfect animals; for a degenerated animal is itself a vitiated production, and how can a vitiated, depraved origin, become a new stock, and not only produce a constant succession of beings, but even to produce them in the same manner, and by following the same laws which produce animals, the origin of which are pure and uncorrupted?
Although we cannot demonstrate that the production of a new species, by degeneration, is a thing impossible in nature, yet the number of probabilities to the contrary render it incredible, for if some species have been produced by degeneration of others, if that of the ass absolutely originated form the horse it can only have happened by a succession of imperceptible degrees, and there must necessarily have been a great number of intermediate animals, the first of which would have differed but slightly in its nature form the horse, and the latter would have approached by degrees to that of the ass."Even if Buffon could not himself believe what he has written here, he had put it into print and it was out and could inspire later naturalists all over the world.