BI 496 — Seminar in Evolution: Darwin and His Critics

On Species and Essentialism

Darwin’s book is about the origin of species. In a rough-and-ready sense, a species is a kind of animal or plant. But does this definition really help, to say that a species is a kind of organism? What does it mean to say that two things are “the same kind,” or that two other things are “different kinds”?

This question—What is a kind or species?—has given rise to some of the most persistent disputes in the history of science and philosophy. The older approach to the question was a view called essentialism. Under an essentialist view of kinds, all things that belong to the same kind—all chairs, all dogs, all humans, all novels, all stars, all horses—share a common essence, while things that belong to different kinds—a dog versus a human—have different essences. You will often see the Greek word εἶδος (eidos, “form” or “type,” cognate with English idea and ideal) used as a synonym for essence. All horses, an essentialist would say, partake of the same essence, an abstract quality we might call “horse-ness,” and that’s what makes them horses. All humans partake of “human-ness” (human nature), and that’s what makes them human. The essence of a kind can be thought of as the ideal model on which all the real examples are based. An essentialist might say that every real chair is just an attempt to capture the idea of perfect “chair-ness.” All real chairs are different, of course, so no chair perfectly captures this essence; every real chair has individual, accidental features that partially mask the essence. But what makes all chairs one kind of thing (what makes them all chairs, as opposed to tables or doors), is that they all embody, however imperfectly, the essence of chair-ness.

The Greek philosopher Plato is one of the most well-known essentialist writers, and his “Allegory of the Cave” from Book VII of The Republic is the most famous early exposition of the essentialist view of kinds. In the Allegory of the Cave, true knowledge is said to be knowledge of the essences of things, not of the varying individual things themselves, because individual variation is the result of accident (local circumstance) rather than essence. (Can you see the biological implications starting to appear?) If you are interested in chairs, an essentialist would say, you should not study particular individual chairs, which may be too short, or too tall, or cracked, or scuffed, or imperfect in some other way; what you should focus your mind on is The Ideal Chair, the essence that makes something a chair. The particular chairs we see around us are like shadows cast on the wall of a cave by The Ideal Chair, which exists in the world of the mind (our mind or God’s mind). The shadows are imperfect and incomplete; true knowledge comes from an understanding of the essence (the form, the idea, the type) that is casting the shadows. An excerpt from the Allegory of the Cave appears below.

Field marks: If you want to spot examples of essentialist thinking in the wild, watch for the definite article: The. When you hear people talking about The Novel or The American Mind or The Human Genome, it’s a fair bet that you’ve hit on an example of essentialist thinking. (There is no such thing as The Human Genome; there are actually six billion different human genomes. And as Heraclitus—the anti-Plato—reminds us, you can never step into the same gene pool twice.)

Essentialism is important to us as evolutionary biologists because it was not only the common way of understanding kinds of inanimate objects, like chairs, tables, and stars, but it was also, before Darwin and Wallace, the common way of understanding kinds of living things (biological species). The second and third quotations below present essentialist pictures of biological species. One is from the American essayist Ralph Waldo Emerson, and the other is from the French naturalist Georges Buffon. Keeping in mind the date of the Origin of Species, notice when these two passages were written. Buffon talks about the eidos of a species as being like an “internal mold” from which each individual is cast during its development (as that individual is “perfected” or completed). Emerson, as an American neo-Platonist, expresses a poetical view of essentialism while trying at the same time to recover some value for individual lives, lives that under a strictly essentialist view are nothing more than shadows of “the perfect Adam.”

The modern evolutionary picture of biological kinds (species), deriving from Darwin and Wallace, is sometimes called population thinking, and it rejects the idea of essentialism entirely. We will discuss population thinking further in class; for the moment, just take note of how the title of one of the two foundational papers of evolutionary biology—Wallace’s 1858 essay, with my emphasis added—is a direct repudiation of essentialism: “On the tendency of varieties to depart indefinitely from the original type.” An essentialist would agree that varieties can depart from the type, but would say that they have a built-in tendency that eventually causes them to “snap back”: to revert to type. Wallace says no: varieties can depart indefinitely; there’s nothing pulling them back.

A thought experiment: Those of you who are philosophically inclined will note that essentialism, with respect to biological species, isn’t false analytically: it’s false empirically. It just isn’t how the world happens to be. One could imagine an alternative universe in which essentialism was true with respect to biological species. In such a universe, what might we have found when we eventually came to understand the distinction between genotype and phenotype and worked out the structure of DNA? Would evolution be possible in such an alternate universe?

Extract I: Plato’s “Allegory of the Cave”

From Book VII of The Republic by Plato (about 360 BC), translated by W.H.D. Rouse:

[Plato, the teacher, says,] “take the following parable … as a picture of the condition of our nature. Imagine mankind dwelling in an underground cave with a long entrance open to the light across the whole width of the cave; in this they have been from childhood, with necks and legs fettered, so they have to stay where they are. They cannot move their heads round because of the fetters, and they can only look forward, but light comes to them from a fire burning behind them higher up at a distance. Between the fire and the prisoners is a road above their level, and along it imagine a low wall has been built, as puppet showmen have screens in front of their people over which they work their puppets.”

“I see,” he [Glaucon, the student] said.

“See, then, bearers carrying along this wall all sorts of articles which they hold projecting above the wall, statues of men and other living things, made of stone or wood and all kinds of stuff, some of the bearers speaking and some silent, as you might expect.”

“What a remarkable image,” he said, “and what remarkable prisoners!”

“Just like ourselves,” I said. “For, first of all, tell me this: What do you think such people would have seen of themselves and each other except their shadows, which the fire cast on the opposite wall of the cave?”

“I don’t see how they could see anything else,” he said, “if they were compelled to keep their heads unmoving all their lives!”

“Very well, what of the things being carried along? Would this not be the same?”

“Of course it would.”

“Suppose the prisoners were able to talk together, don’t you think that when they named the shadows which they saw passing they would believe they were naming things?”

“Necessarily.”

“Then if their prison had an echo from the opposite wall, whenever one of the passing bearers uttered a sound, would they not suppose that the passing shadow must be making the sound? Don’t you think so?”

“Indeed I do,” he said.

“If so,” said I, “such persons would certainly believe that there were no realities except those shadows of handmade things.”

“So it must be.”

. . . . . . . . . .

“Then we must apply this image, my dear Glaucon,” said I, “to all we have been saying. The world of our sight is like the habitation in prison, the firelight there to the sunlight here, and the ascent [through the tunnel and out of the cave] and the view of the upper world is the rising of the soul into the world of the mind.”

A literary aside: Plato’s Cave has been an object of literary reference for more than 2000 years. When Emily Dickinson writes about “Races nurtured in the dark,” what do you suppose she has in mind? A piece of chocolate will go to anyone who comes up with similar cavernous allusions.

Extract II: Buffon on the “Internal Mold”

From Georges Buffon’s Histoire Naturelle (Paris, 1743), quoted by Philip R. Sloan in “From logical universals to historical individuals: Buffon’s idea of biological species,“ one of several papers in the volume Histoire du concept d’espece dans les sciences de la vie, edited by Scott Atran (Paris, 1987):

There is, in nature, a general prototype in each species upon which each individual is modeled, but which seems, in realizing itself, to be altered or perfected by circumstances. So that, relative to certain characteristics, there is an unusual variation in appearance in the succession of individuals, and at the same time a constancy in the species as a whole which appears remarkable. The first animal, the first horse, for example, has been the external model and the internal mold upon which all horses which have ever been born, all those which now exist, and all which will arise, have been formed. But this model, which we know only by its copies, has been able to be altered or perfected in the communication and multiplication of its form. The original impression subsists in its entirety in each individual, but although there might be millions of them, none of these individuals is similar in entirety to any other, nor, by implication, to the impressing model.

Extract III: From Emerson’s “Nominalist and Realist”

The verse epigraph to Ralph Waldo Emerson’s 1844 essay “Nominalist and Realist”:

In countless upward-striving waves
The moon-drawn tide-wave strives;
In thousand far-transplanted grafts
The parent fruit survives;
So, in the new-born millions,
The perfect Adam lives.
Not less are summer-mornings dear
To every child they wake,
And each with novel life his sphere
Fills for his proper sake.

“Nominalist and Realist”: The terms nominalist and realist in Emerson’s title refer to those who regard essences as real (essentialists or realists) and those who deny essentialism and regard essences as nothing more than common nouns (abstract names rather than things). Population thinking is closer to nominalism than it is to realism (essentialism), but it’s better thought of as a tertium quid—a third category.

Essence and accident: If you want a good history-of-ideas project, take a look at another neo-Platonic Emerson essay, the rarely-read “Illusions.” I could be wrong, but I feel certain that some of its imagery alludes to the Origin of Species. (Who is the “sad-eyed boy ... afflicted with a tendency to trace home the glittering miscellany of fruits and flowers to one root“?) If “Illusions” doesn’t point to the Origin, it must point to some other early evolutionary writings.

Additional References for the Philosophically Inclined

If you find the topic of essentialism and population thinking to be especially interesting, here are three more references you can track down:

• Gelman, Susan A. 2003. The Essential Child: Origins of Essentialism in Everyday Thought. Oxford: Oxford University Press.
• O’Hara, Robert J. 1997. Population thinking and tree thinking in systematics. Zoologica Scripta, 26: 323–329.
• Sober, Elliott. 1980. Evolution, population thinking, and essentialism. Philosophy of Science, 47: 350–383.