In the course of his examination of the animal world, Buffon noted that despite similar environments, different regions have distinct plants and animals, a concept later known as Buffon's Law. This is considered to be the first principle of biogeography. He made the suggestion that species may have both "improved" and "degenerated" after dispersing from a center of creation. In volume 14 he argued that all the world's quadrupeds had developed from an original set of just thirty-eight quadrupeds. On this basis, he is sometimes considered a "transformist" and a precursor of Darwin. He also asserted that climate change may have facilitated the worldwide spread of species from their centers of origin. Still, interpreting his ideas on the subject is not simple, for he returned to topics many times in the course of his work.