Speciation – A brief history: The late eighteenth century

After Linnaeus had settled on the older mechanism of hybridisation of genera with other genera or with varieties formed by geographical conditions as the cause of new species, the topic began to pick up speed. Hybridisation remained the usual method as late as the 1830s (e.g., in Lindley) but two developments were crucial in the rise of the “species problem” of the nineteenth century.

One was to do with the degree of freedom in hybrids. Linnaeus had originally held that all species that could exist did, and only later in his work, after around 1751, did he allow for there to be as-yet unfilled possibilities. He had held that all species formed a continuum, like countries with adjacent borders. In his later work, he permitted some regions of the “species landscape” to be unfilled, which hybrids were able to enter. As Peter F. Stevens (1994) has shown, with Antoine Laurent Jussieu (1748–1836) in the French tradition, these regions became more sparsely inhabited. Mechanisms for new species in the uninhabited regions were called for. A “natural system” – one in which the relations were objective (“in nature”) – was called for. But most, such as Adanson and Jaume Saint-Hilaire (1772–1845), still believed that when we had found all species there would be no gaps (Stevens 1994:95ff).

The other, however, was the introduction of evolutionary, or transmutationist, ideas. Linaneus’ Systema Naturae introduced the notion of species fixity when it was first published in 1735. A mere decade later, the first transmutationist notion of species was published by Pierre Maupertuis, a physicist who also discovered something like the Mendelian ratio. In his Venus Physique (translated as The Earthly Venus) in 1745, Essai de cosmologie, in 1750, Maupertuis wrote:

Could one not say that, in the fortuitous combinations of the productions of nature, as there must be some characterized by a certain relation of fitness which are able to subsist, it is not to be wondered at that this fitness is present in all the species that are currently in existence? Chance, one would say, produced an innumerable multitude of individuals; a small number found themselves constructed in such a manner that the parts of the animal were able to satisfy its needs; in another infinitely greater number, there was neither fitness nor order: all of these latter have perished. Animals lacking a mouth could not live; others lacking reproductive organs could not perpetuate themselves… The species we see today are but the smallest part of what blind destiny has produced… [translation from Glass 1959:58]

In another work he wrote in 1751:

Could we not explain in this manner [of fortuitous changes] how the multiplication of the most dissimilar species could have sprung from just two individuals? They would owe their origin to some fortuitous productions in which the elementary parts [of heredity] deviated from the order maintained in the parents. Each degree of error would have created a new species, and as a result of repeated deviations the infinite diversity of animals that we see today would have come about. [Systèm de la Nature 2:164, quoted in Terrell 2002:338]

Maupertuis’ idea is a revised version of Empedocles’ lucky monsters theory, but it differs in that the parts which recombine are heritable variations of traits caused by blind chance based on what prior parental traits were inherited with “error”. There is no real idea of population or competition, but it is at least in the direction of natural selection.

Later evolutionary (or better, transformist) views included limited varieties such as that of Buffon, through to the universal transformism of Erasmus Darwin or Lamarck. One author was Denis Diderot, whose theory, or better passing comment, is this:

It seems that nature has taken pleasure in varying the same mechanism in a thousand different ways. She never abandons any class of her creations before she has multiplied the individuals of it in as many different forms as possible. When one looks out upon the animal kingdom and notes how, among the quadrupeds, all have functions and parts—especially the internal parts—entirely similar to those of another quadruped, would not any one readily believe (ne croirait-on pas volontiers) that there was never but one original animal, prototype of all animals, of which Nature has merely lengthened or shortened, transformed, multiplied or obliterated, certain organs? Imagine the fingers of the hand united and the substance of the nails so abundant that, spreading out and swelling, it envelops the whole and in place of the human hand you have the foot of a horse. When one sees how the successive metamorphoses of the envelope of the prototype—whatever it may have been—proceed by insensible degrees through one kingdom of Nature after another, and people the confines of the two kingdoms (if it is permissible to speak of confines where there is no real division)—and people, I say, the confines of the two kingdoms with beings of an uncertain and ambiguous character, stripped in large part of the forms, qualities and functions of the one and invested with the forms, qualities and functions of the other—who then would not feel himself impelled to the belief that there has been but a single first being, prototype of all beings? But whether this philosophic conjecture be admitted as true with Doctor Baumann [Maupertuis*], or rejected as false with M. de Buffon, it can not be denied that we must needs embrace it (on ne niera pas qu il faille I’embrasser) as a hypothesis essential to the progress of experimental science, to that of a rational philosophy, to the discovery and to the explanation of the phenomena of organic life. [Denis Diderot, 1753, Pensées sur l’interpretation de la nature, ch. XII, as translated by Lovejoy 1904: 325]

In other words, things evolve over time, but there are no divisions between them, so any classification into species is arbitrary. Hence there can be no “origin of species”, unlike Maupertuis’ views. Diderot here represents the views of the Encyclopedists, of whom he was a leading member as the chief editor of the Encyclopédie. A similar view was espoused earlier by Charles Bonnet (1720–1793), who arrayed species along a continuum from simplest to most complex. Bonnet’s vision was, however, static rather than transformist. Reaction to his scheme was mixed. One rejection was by Peter Simon Pallas, in Elenchus Zoophytorum (1766) , where he argued that we had to represent living things as a branching tree. Again, Pallas was no transformist, but he laid the ground for something like tree thinking, which became the problem Darwin tackled.

Maupertuis’ idea of fitness being what remains after chance has caused variations was not regarded as terribly likely, and on the point of atheism by many. Apart from the hybrid theory, and this, however, there was another idea, not unlike Linnaeus’. That is, speciation by geographical variation. The key figure here is Buffon.

Buffon

Georges Louis Leclerc, comte de Buffon (1707–1788) was an influential figure in the eighteenth century. He tended the King’s Garden (Jardin du Roi, later the Garden of Plants, Jardin des Plantes) and wrote a multivolume series entitled Natural History (Histoire Naturelle 1749–1788, in 36 volumes) in which he appeared to post-Darwin scholars to be given an evolutionary theory. In an early volume (1751: “The Ass”) he wrote that species today are “nearly” what they were when created, and that fertility between individuals is what makes a species:

… we can draw a line of separation between two species, that is, between two successions of individuals who reproduce, but cannot mix; and, as we can also unite into one species two successions of individuals who reproduce by mixing.  This is the most fixed and determined point in the history of nature. All other similarities and differences which can be found in the comparison of beings, are neither so real nor so constant.

Later, however, he seemed to present a transformist view: species were not what Linnaeus called species but more like what came to be known as “families”. All canine species formed a group that degenerated from the original dog, all feline species likewise, along with horses, sheep, and cattle. The causes of this degeneration were local climatic and geographical influences upon the seminal fluid and its expression. In the initial essay, he argued that the Ass was not a degenerate horse, because it was infertile when crossed with horses. Later, in “Of the degeneration of animals” (1766), he called the ass a degenerate form of the horse, along with the zebra. As Farber notes (1975: 69):

What Buffon envisioned was the production, in time, of natural genera created by the gradual modification of species. For each genus or family (Buffon used the two words interchangeably) there was an original premier souche [primary stock] that had degenerated into several recognizably different varieties, what we commonly call species. One could show that all the individuals came from a common ancestor by noting their morphological similarity and their alleged ability to interbreed.

This was due to an interior mould (moule intérieur) that caused the embryo to develop into the adult organism; under different conditions, that mould would be expressed differently. Thus a genus or family was the “true” species, and our “species” but varieties of the primary stock. We can classify them all as related because they are the constant forms, despite their outward differences. In modern terms we would call this developmental systems modified by epigenetics (but note: epigenetics means something very different in the period under consideration than it does today. Here it means that the embryo is formed through a developmental process rather than being preformed in the seminal fluid).

Buffon, being a Lockean empiricist as well as a Lockean nominalist, even did a number of cross breeding experiments, with some success, to prove his theory of degeneration. While he got little result from crossing pigs and peccaries, dogs and wolves, dogs and foxes, or hares and rabbits, he did get results from goats and sheep (Roger 1997: 319). Unlike Linnaeus, his vision of hybridisation between “species” in the Linnaean sense was regressive rather than the origin of new species.

Next, I shall discuss the eighteenth century evolutionists Erasmus Darwin and Lamarck.

References

Farber, Paul Lawrence. 1975. “Buffon and Daubenton: Divergent traditions within the Histoire naturelle.” Isis 66 (1):63-74.

Glass, Bentley. 1959. “Maupertuis, pioneer of genetics and evolution.” In Forerunners of Darwin 1745-1859, edited by Bentley Glass, Oswei Temkin and William L. Straus, 51-83. Baltimore: Johns Hopkins.

Lovejoy, Arthur O. 1904. “Some eighteenth century evolutionists. II.” Popular Science Monthly LXV (August):323–340.

Roger, Jacques. 1997. Buffon: A life in natural history. Translated by Sarah Lucille Bonnefoi. Edited by L Pearce Williams, Cornell History of Science Series. Ithaca, NY: Cornell University Press.

Stevens, Peter F. 1994. The development of biological systematics: Antoine-Laurent de Jussieu, nature, and the natural system. New York: Columbia University Press.

Terrall, Mary. 2002. The man who flattened the earth: Maupertuis and the sciences in the enlightenment. Chicago: The University of Chicago Press.

Notes

*. Maupertuis initially published Système de la Nature, based on his PhD dissertation, under the pseudonym of Dr Baumann.