Darwin on species, 1

Recently, John Lynch mentioned a short passage in a book by historian Peter Dear, called The intelligibility of nature. Dear wrote this:

It is one of the remarkable facts about nineteenth-century natural history that the practices of taxonomists were not thrown into disruption by the eventual publication and acceptance of Darwin's ideas after 1859. Darwin was to rely on taxonomy for much of his argument in Origin of Species, reinterpreting its meaning in terms of the branching theory of descent. He never paused to ask whether the very meaning of the category "species" might have been radically changed by his theory, in such a way that earlier taxonomic practices would have to be called into question. It would not have been in his interests to do so, because existing taxonomy provided him with valuable arguments. Taxnomists demonstrated lines of filiation between species; Darwin would simply explain what these connections meant in a new and, he hoped, more convincing way. [p96]

Now, I do not intend to discuss Dear's overall thesis, as I don't have the entire book. But this is egregiously wrong. It is false in almost every important respect. And Dear is not alone in this sort of misunderstanding about Darwin and species. Something like it is stated authoritatively by almost every evolutionary textbook or book on species that appeals to history. It's time to defend Darwin, and look a bit more carefully at what turn out to be rather sophisticated discussions by Darwin for his time.

Let us state at the outset - Darwin was very concerned about the category of "species", and one of his major claims, both in print and in his correspondence with scores of taxonomists, was that evolution is based on a radical claim about species: they are more or less permanent varieties, rather than constraints on the limits of variation. And as for taxonomic practice, well we'll get to that a bit later.

To begin with, what was the standard interpretation of species in the early nineteenth century? Don't believe what you may have read, that all taxonomists were "morphologists" or "typological". It simply isn't true, or rather, in the sense it is true, so was Darwin and so is every taxonomist today, and in the sense in which people want to make a claim about pre-Darwinian taxonomy, it is simply false.

The basic idea of species in natural history, and the then nascent discipline of Biology, was this, as defined by Cuvier:

My research assumes the definition of species which serves as the basic use made of the term, understanding that the word species means the individuals who descend from one another or from common parents and those who resemble them as much as they resemble each other. [Règne Animal, i, p19, 1812]

This definition, made in passing to combat the species denial, or nominalism, of Lamarck, who Cuvier despised, was enormously influential. Notice that it has two parts: that individuals descend from parents, and that they do so resembling them as much as they resemble their conspecifics. A similar and much earlier definition, also widely cited, was John Ray's, from 1686:

In order that an inventory of plants may be begun and a classification of them correctly established, we must try to discover criteria of some sort for distinguishing what are called "species". After long and considerable investigation, no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species ... Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa. [Historia plantarum generalis, italics added]

Notice what the italicised definition (often quoted in Latin as propagatio ex semine) says. It, too, has a dual aspect - features + propagation [from seed, equivalent to Cuvier's "parents"]. In a review of the history of biological species conceptions all the way back to Aristotle, I find that form and generation, or morphology and reproduction, are constantly conjoined together whenever someone talks about species, or any other similar word for the basic kinds of organisms. I call this the Generative Conception of Species. While form, or resemblance, or appearance (species and its Greek cognate eidos both mean "seeming" as well as form) is indeed part of the general understanding of species right up until the nineteenth century, so too is reproduction. It's not enough that they look the same, but they have to be able to reproduce as well. That is the essence of a species - that it has the generative power to reproduce the same form.

Cuvier and Ray set the backdrop to debates in the nineteenth century that came to be called "the species question". But it's important to understand that the "question" was not about the nature of species, but about their origin, and despite what Dear says above, Darwin did not frame that question - it was widely discussed by botanists and zoologists, and at least one geologist, well before him.

You see, it was widely understood that flora and fauna changed over time. Cuvier himself had written a popular book on the topic (Discours sur les révolutions du globe, or Discourse on the Revolutionary Upheavals on the Surface of the Earth, 1812). So it was an open question - where did new species come from? There were all kinds of speculations, including, let it be said, various kinds of evolutionary, or rather, transmutationist, accounts. Species came from prior species, changed. A dangerous view that was tied into radical politics, particularly in Britain and France.

What about taxonomic practice? It's important too, to distinguish between the taxonomy performed in the field and by dissection and microscopy work, from that done in museums on dried, skinned or preserved specimens. Given the limitations on travel and preservation (no refrigerators), museum specimens were described mostly by the local anatomist who worked in those organisms. Botanical specimens were dried and pressed. The field worker drew the specimen in situ if they were able, and the data was assembled in the home museum, sometimes years later. Specimens with hard parts, like insects and spiders, were a lot more easily identified this way than, say, a highly variable mammal species. In those cases, one needed to observe them in their habitat if there was too much variation.

And variation had, by the 1830s, been recognised as a major source of difficulty in this sort of taxonomy. Alphonse de Candolle, a famous French botanist, had made quite a study of it, and the message was getting out. But the taxonomic practice of the museum taxonomist was to take a single specimen (the type specimen or as we now call it, the holotype) and describe the species from that. It was convenient to do so, but did not rely on any underlying notion of what species were, although there were some implicit assumptions about variation and likely differences between sexes.

Another tradition that was in play at the time apart from the Ray-Cuvier definitions was the view of Buffon, who in the context of a massive and popular series on natural history, defined species as arbitrary constructs due more to our own limitations than the boundaries of nature:

The error consists in a failure to understand nature's processes (marche), which always take place by gradations (nuances). ... It is possible to descend by almost insensible degrees from the most perfect creature to the most formless matter. ... These imperceptible shadings are the great work of nature; they are to be found not only in the sizes and forms, but also in the movements, the generations and the successions of every species. ... [Thus] nature, proceeding by unknown gradations, cannot wholly lend herself to these divisions [into genera and species]. ... There will be found a great number of intermediate species, and of objects belonging half in one class and half in another. Objects of this sort, to which it is impossible to assign a place, necessarily render vain the attempt at a universal system. ...

In general, the more one increases the number of one's divisions, in the case of the products of nature, the nearer one comes to the truth; since in reality individuals alone exist in nature. [Histoire naturelle, (1749), p12, p13, p20, p38

His student Lamarck, repeated this view. But Buffon also defined species in another way, adding to the Generative Conception another criterion - interfertility - in the second volume of the Histoire naturelle:

... if, by means of copulation, they can perpetuate themselves and the likeness of the species; and we should regard them as belonging to different species if they are incapable of producing progeny by the same means. Thus the fox will be known to be a different species from the dog if it proves to be a fact that from the mating of a male and female of these two kinds of animals no offspring is born; and even if there should result a hybrid offspring, a sort of mule, this would suffice to prove that fox and dog are not of the same species - inasmuch as this mule would be sterile (ne produirait rien). For we have assumed that, in order that a species might be constituted, there was necessary a continuous, perpetual and unvarying reproduction (une production continue, perpétuelle, invariable) - similar, in a word, to that of other animals.

The question whether species were defined by fertility was henceforth on the table, and especially in European biology it was often appealed to as a way of telling if two forms, or "morphs" were conspecific.

A rather funny passage appears in an unpublished manuscript of Flaubert's, left unfinished at his death, of a story of two men who win the lottery and decide to eat everything they can, in true French cuisine, so they decide to breed new animals:

They opened Buffon again and went into ecstasies at the peculiar tastes of certain animals.

...

They wanted to try some abnormal mating. ...

They made fresh attempts with hens and a duck, a mastiff and a sow, in the hope that monsters would result, but quite failing to understand anything about the question of species. This is the word that designates a group of individuals whose descendants reproduce, but animals classified as different species may reproduce, and others, included in the same species, have lost the ability to do so. [Flaubert, Bouvard and Pécuchet, with the Dictionary of Received Ideas, 1976: 87]

Although this is late, around the 1880s, it is a nice summary of the confusion about species in this tradition.

In the next post in this series, I will cover the young Darwin's thoughts on species as he travelled around the world, and as he formulated his view on evolution, prior to the publication of the Origin in 1859.