If you've ever taken a biology course, or if you've ever cracked a biology textbook, or if you've ever read a popular science book about biology, then you've probably seen some variation on the story given here:"Plato defined Form (eidos) as something that had an essence, and Aristotle set up a way of dividing genera (gene) into species (eide) so that each individual in the species shared the essence of the species. Linnaeus took this idea and made species into constant and essentialistic types. Darwin overcame this essentialism." (From Species: A History of the Idea by John Wilkins)
I have two introductory-level textbooks in biology, and both give this same story, in its essence. This is the received view of the history of biology. I'm going to put aside Plato--for a variety of reasons, not least of which being that anyone who lumps Plato in with Aristotle clearly hasn't read Plato or Aristotle--so we can summarize: Aristotle leads to Linnaeus and then there's a clean break when Darwin comes along.
There's a problem with this story: it just doesn't work, for a number of reasons that we'll soon see. You could even say that the only thing it gets right is that there were people named Aristotle, Linnaeus, and Darwin. Since these are the main characters with whom we'll be dealing, I'll just say a few words about each of them.
You've all taken at least some philosophy, so the name "Aristotle" should be familiar by now: ancient Greek; student of Plato; teacher of Alexander. For a thousand years he was considered the only philosopher worth reading, and even since then he's widely recognized as the first true biologist.Carl von Linne, or Carolus Linnaeus as most now know him, was a Swedish botanist in the eighteenth century. He's responsible for the naming system used in biology today, wherein organisms are categorized into a hierarchy of groupings. He was also arrogant enough to change his name, which he didn't think was impressive enough, to a Latin name that sounded better.
We've already mentioned Darwin, about whom I'll have more to say later.
Let's start, as the received view does, with Aristotle. As I said, he's widely considered the first biologist; in particular, he's credited with developing the first system for diving up the natural world. He was particularly concerned with how things should be defined and he developed a method for finding such definitions: this method is called "classification by division."
The method is detailed in Aristotle's Posterior Analytics, one of his works on logic. It goes like this: you start with a whole bunch of things in a group, and then you find a way to divide up that group. You look for the qualities that make some parts of the group similar to others, but more importantly, you look for the things that make some parts of the group different from the others. You then pile difference upon difference, and eventually you'll have each part of the whole divided off from the rest; the difference that sets each piece apart is what defines the piece.
Consider, then, the group of canids. Using classification by division, we find that there are certain traits that make some of these dogs different from others in this group. We can therefore divide the dogs off according to these differences:
We can divide the canids into four groups here: wolves, domestic dogs, dingos, and coyotes. Each of these groups is considered a species of the larger whole, which Aristotle called the genus.
Modern biologists looked at what Aristotle did and basically said, "Hey, he's using the same words that I use, so we must be talking about the same things!" There are a couple of problems with saying so. First of all, Aristotle didn't use these words: he spoke Ancient Greek; English wasn't going to be around for another few hundred years. Even so, the way Aristotle used these terms just doesn't track with the way we use them today. Consider, for example, that in his Parts of Animals Aristotle refers to domestic dogs as a species, but in his History of Animals he refers to domestic dogs as a genus. In other words, the very same group could be considered either a species or a genus, depending on what's being divided up.
So we can divide wolves up into gray wolves and Arabian wolves; domestic dogs into pugs and huskies. In each case, what was formerly a species become a genus that includes other species. These subordinate species don't have to track with what we would normally consider to be genuine biological kinds: for example, male dingos and female dingos would each be species of the dingo genus while adults and babies would be species of the coyote genus. This isn't an isolated case, either: this is one of at least five examples wherein Aristotle does the same thing with various animal groups. He draws similarly relativistic divisions within birds and three other groups that escape me right now.
This just isn't how biologists now divide up the world. If anything, the distinctions that Aristotle intends to draw using classification by division aren't biological in nature; they're logical distinctions. After all, the Posterior Analytics is a book about logic.
Whatever the case, this is Aristotle's classification by division: we start with a larger group, then divide is further and further down. Now, according to the received view, Linnaeus is supposed to be a disciple of Aristotle. Let's take a look, then, at what Linnaeus has to say about the method of classification by division:
"...by these hypothetical and arbitrary principles they broke and tore apart the natural, non-arbitrary genera and did violence to nature." (From Ratio Operis by Linnaeus, aphorism 8)
"Broke and tore apart!" "Did violence to nature!" These are not the words of a disciple! I mean, with disciples like this, who would need enemies?
Linnaeus thought that the divisions between species and genera were entirely real and that our classifications should track with these divisions. The problem with Aristotle, in his view, was that classification by division can continue dividing up even the natural groups. Worse, in Linnaeus's opinion, was that the way Aristotle used the concepts "genus" and "species" were arbitrary: a genus could just as easily be a species, and vice-versa, depending on context. Linnaeus didn't like that.
In a way, the method that Linnaeus used for classifying organisms is the polar opposite of Aristotle's method. Linnaeus called it "collation of species."
In collation of species, rather than dividing from the top down, we build from the bottom up. Linnaeus proposed that we take a look at all the various organisms out there and set aside the smallest group of similar organisms that we could find. This smallest group would be the species.
Someday, Natalie...
It's interesting to note that the way Linnaeus defined the species group was very similar to the way Mayr did: Linnaeus suggested that greatest similarity was determined by the ability to interbreed. However you define species, though, once the species is defined we can then find species groups that are most similar to one another and group those together into a genus.
And we can then find the genera that are most similar to one another and group those together into orders, and so on and so forth.
We've therefore built the whole system from the ground up. This made for an important difference from Aristotle's method: each category is absolute. A species is a species is a species, regardless of the context, and the same goes for genera and orders and classes and kingdoms.
These are the only five divisions that Linnaeus himself recognized. These days, biologists recognize seven or eight divisions, depending on which source you consult. The current divisions are the Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. The mnemonic device that my undergrad biology professor suggested for this system is "diligent kindly professors cannot often fail good students," which strikes me as an open empirical question.
Anyway, we have another problem: if, as the received view says, Darwin threw the Linnaean trash out, then why are we still using this system, let alone building on it?
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