Monday, October 12, 2015

Buffon and the origin of the tree and network metaphors

I have written before about Georges-Louis Leclerc, Comte de Buffon (1707-1788). (Actually, he was called Georges-Louis Leclerc from 1707-1725, and Georges-Louis Leclerc De Buffon from 1725–1773, before becoming a count.) His role in the development of the theory of organic evolution was such that he is worth considering again here, especially given his important role in introducing the tree and network metaphors in phylogenetics.


Buffon is usually credited with being in the top triumvirate of influential people in the development of modern biology, along with Aristotle and Darwin. Buffon followed the lead of the physicist Isaac Newton, by trying to explain natural phenomena solely in terms of other observable natural phenomena, rather than resorting to super-natural explanations. (Indeed, Buffon translated one of Newton's books from LAtin to French.)

This was Newton's main contribution to science, his insistence on empirical explanations. He did not invent this idea, but he was the one who effectively created modern science by consistently applying it. Hence the importance of the apple — the explanation for the small-scale phenomenon of a falling apple, which we can see and study experimentally, is the same as for the large-scale orbits of the planets, which we can see but not experiment upon. Consistency of natural explanations, rather than invoking super-natural forces, creates a coherent scientific whole that is amenable to description, explanation and prediction.

Buffon adopted this same scientific approach and applied it to biology. Once again, he did not invent this idea, but he was the one who applied it consistently across all of biology. He did this principally in his Histoire naturelle, générale et particulière, an ambitious work planned to cover all of nature in 50 volumes (it included geology, anthropology and cosmogeny, as well as biology). Begun in 1749, he and a few collaborators completed 36 volumes before his death in 1788, and 8 more were compiled by others shortly afterwards.

In the process of trying to find natural explanations for all empirically observable biological phenomena, Buffon not unexpectedly encountered the idea of mutation of species, as part of his thoughts about an irreversible history of nature. He thus grappled both with species concepts and with temporal change within and between species. He is thus credited as the first modern evolutionist, because he introduced the time element in comparative biology, so that common structure is explained in terms of common ancestry. However, his ideas, published over many decades, were often inconsistent — sometimes he was an evolutionist and sometimes not. This seems to be, at least in part, due to increasing religious pressure — he was an important person in the ancienne regime of France, and not in a position to easily reject the teachings of the Catholic church.

By modern standards, Buffon was wrong on most things (see Buffon's genealogical ideas), as was Aristotle — being first means that you are also the first to get it wrong, to one extent or another. This does not in any way reduce the impressive nature of his work as a pioneer. He was not a cataloguer of information like his great Swedish rival von Linné — he wanted to explain things, not organize them, as he was interested principally in causes. He also moved away from trying to explain biology in terms of physics (eg. the concept of universal essences), and tried to explain it in terms of itself.


Of principal interest for this blog is Buffon's role in the development of metaphors for biological relationships. Given his role as an early adopter of evolutionary ideas, he was also an early adopter of metaphors to depict those ideas about historical relationships.

Buffon argued for temporal continuity rather than eternal types, modification of both natural and domesticated species through time (but only up to a certain point), and an underlying unity of organismal types. The latter idea suggested common ancestry for all animals, but Buffon considered and rejected this hypothesis. Indeed, he also rejected the idea that species descend from each other, thus accepting only within-species evolution. He did, however, have a broad concept of species, based on inter-breeding, so that some of his species correspond to modern taxonomic families.

In a previous blog post (The first phylogenetic network 1755) I noted that Buffon put his thoughts into action when he considered the within-species evolution of dog breeds in volume V his Histoire naturelle. In doing so, he published what is usually considered to be the first avowedly evolutionary diagram. It shows the origin and diversification of dog domestication as known at the time. It includes both temporal and spatial variation among dogs, since Buffon believed that morphological variation was related to different climates, so that climatic differences were the ultimate cause of biological variation.

Although Buffon labeled the diagram as a "Table", in his text he noted that it is [translated] "a table or, if one prefers, a kind of genealogical tree where one may grasp at a glance all the varieties". In modern terms it is actually a hybridization network, since it shows repeatedly that some dog breeds arose as a result of hybridization between other breeds. It is also, of course, a map, since it shows spatial variation, although the geographical content is not strictly respected. The diagram is thus a hybrid of a network and a map.

Note that Buffon used the idea of a tree long before Simon Pallas (1776), who is usually credited with introducing the tree metaphor. However, Buffon was writing solely about within-species relationships, whereas Pallas discussed a much broader scale (specifically, both plants and animals).

Indeed, Buffon's genealogical ideas had first appeared in volume IV of the Histoire naturelle, in 1753 (the same year as Linné's Species Plantarum). In this volume there is a presentation of his ideas on species in "Discours sur la nature des animaux" [Discourse on the nature of animals] and his ideas about animal genealogy in "L'asne" [The ass]. The latter contains this text:
que l'homme et le singe ont eu une origine commune comme le cheval et l'âne; que chaque famille, tant dans les animaux que dans les végétaux, n'a eu qu'une seule souche, et même que tous les animaux sont venus d'un seul animal qui, dans la succession des temps, a produit, en se perfectionnant et en dégénérant, toutes les races des autres animaux. [that man and ape have had a common origin like the horse and the donkey; every family, both in animals and in plants, had only a single stem [stock], and even all the animals came from a single animal which, in the succession of time has produced by perfection and degeneration, all the races of the other animals.]
Buffon was, however, not consistent in his uses of metaphors. This topic is discussed in detail by Giulio Barsanti (1992), and he has provided a convenient chart of Buffon's metaphors — the following version is taken from Ruse and Travis (2009).

Note that Buffon used the traditional chain analogy most often, since this can be used for ancestor–descendant relationships. However, he simultaneously used the tree and map in 1755 (as discussed above), and he effectively replaced the tree with the map after 1780. The map had previously been introduced by von Linné in 1751 ("All plants show affinities on either side, like territories in a geographical map").

It is interesting to see the rapid rise and fall of the family-tree metaphor in the mid 1700s, before its resurgence a century later. The cluster of tree references in 1766 is from "De la dégénération", in volume XIV of Histoire naturelle. "Dégénération" was Buffon's term for evolution.


Barsanti G (1992) Buffon et l'image de la nature: de l'échelle des êtres à la carte géographique et à l'arbre généalogique [Buffon and the image of nature: the scale of being to the map and to the family tree]. In: Gayon J (ed.) Buffon 88: Actes du Colloque International [pour le bicentenaire de la morte de Buffon] (Paris-Montbard-Dijon, 14-22 juin 1988), pp. 255-296. Paris: Librairie Philosophique J. Vrin.

Ruse M, Travis J (2009) Evolution: The First Four Billion Years. Belknap Press, Cambridge MA, p 458.

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