Several things forced themselves on my thinking as I read what people had said about species before Darwin. I will try to outline them.
First, people were not stupid or bad observers, before Darwin, and they did not suddenly become smart and good observers the day he published the Origin. People knew that species were variable as far back as I can find. Especially just before the rise of modern biology, but even as far back as the 12th century, you find good observers noting that (for instance) two kinds of hawk are the same because they can interbreed, despite differences in plumage and form. This is Frederick II of Hohenstaufen, Holy Roman Emperor and mad keen falconer, a sport that the west had recently adopted from the Muslim aristocracy. It is said that Frederick spent equal amounts of time working on the affairs of state as on his falconry, and he owned over a thousand birds. In writing a book, The Art of Hunting with Birds, for breeders and trainers, he noted that Aristotle was insufficient, because he was credulous and used hearsay. Frederick later gave access to his falconers to Albert the Great, Thomas Aquinas's teacher, and Albert also managed to give a pretty good account of actual species.
Mind, until 1686 the word 'species' was just a vernacular Latin word. It had no more special significance than if I say of a spanner, that it is a kind of spanner. Species did have technical meanings in logic and in theology, but if I said in Latin that there was a kind of bird outside I was not committing myself to saying that the kind had necessary and sufficient properties, as one might in logic. So the second point I want to make is that the essentialist story confuses the logical meaning of species with the later biological meaning. In fact, naturalists who studied organisms almost never refer to the logical sense, and when they do, they are careful to distinguish the two meanings. In a well-known introduction to logic in the early 19th century, before Darwin even went to university, Archbishop Richard Whately observed that the logical meaning and the naturalists' meaning of species were technically different. Right up to the First World War, logicians would repeat this obvious and simple claim. Unfortunately, the essentialist story authors missed these caveats even in the source texts they used as evidence.
Naturalists were typologists, however. Types and logical classes are quite different things. A botanist might designate a type species for a genus of flowering plant, knowing full well that not all species in it would have the 'typical' traits used to 'define' the genus. So long as most roses had most of the traits of the 'type' Rose, they were roses. Types were exemplars that one used to organise things around in your classifications. The leading historian of the science of classification, Polly Winsor, has called this 'the method of exemplars', and it was not only pre-Darwinian, it was the same method used by Darwin and every taxonomist until the modern era. In fact, Darwin was on the committee that established Linnaean taxonomy as the way to do biological classification.
My third point is that the idea that species were fixed at creation was not the default view before Darwin, and it did not, as the essentialist story holds, begin with Aristotelian thinking in the western tradition. Fixism, as we now call it, was not asserted until non-biologists like the Jesuit theologian and polymath Athanasius Kircher in the 17th century tried to explain how Noah's Ark could hold all living things. This work influenced a young botanist named John Ray, and in 1686, in the course of publishing a series of botanies and zoological works of his late friend, Francis Willughby, he first defined what he meant in natural history by the Latin word 'species', and I quote:
"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."
Ray did not defend this claim that species were fixed. It was simply a matter of piety. He was one of the founders of the British tradition in natural theology, the project to find information about God through the study of the natural world. When Linnaeus wrote in 1736 that 'There are as many species as the Infinite Being produced diverse forms in the beginning', he was merely repeating what he had read in Ray.
However, before Ray, and all the way back to Aristotle, species were malleable things, generating monsters and hybrids that bred true, and given that there was no biological meaning to the term or concept before Ray, to say that people then were species fixists is to commit an anachronism. In any case, Aristotle talked of hybrids forming new animals in Africa and India, a claim that was repeated not only by Pliny, the main source of natural history for 1,600 years, but by at least one of the translators of the King James Bible. And even Linnaeus himself said that species could form by hybridism. As a side note, let us observe that Aristotle didn't even use the Greek term for 'species', eidos, when describing animals and plants.
My fourth point is that there is only one concept of species, and it is one that goes back at least to Epicurus, Aristotle's younger contemporary, if not to Aristotle himself. Plato, of course, was not in the slightest interested in the mundane world, so we can ignore him. I call this the generative conception of species and it is implicit in Ray's definition: A species is when progeny resemble parents due to the generative powers they share. In short, species are reproduction plus resemblance. Even Darwin, and I would say, all modern biologists, hold to this conception. What modern biologists disagree over is how to explain this; what it means to generate similar morphologies.
The term 'morphology' in biology means the overall shape or appearance of an organism, together with internal resemblances of physiology and anatomy. The essentialism story authors, like Ernest Mayr, thought that there was a 'morphological definition' of species, but really this was just a method of diagnosing museum specimens. Again, Darwin used exactly this method, and helped formulate it. Diagnosis, however, is not the same as the thing being diagnosed, or broken bones would consist in part in having X-rays. Even today, taxonomists occasionally confuse the naming and diagnosis of species with the species themselves. If a fly is diagnosed by a single cross-vein in its wings, that is not what makes it a species, of course, and nobody ever thought that it did, before Darwin, or after. The morphological definition is a phantom.
One of the interesting things about the species debate is how scientists use history to bolster their own views or undermine their opponents'.
Darwin is such a crucial figure in the development of modern biology that the temptation is constantly there to claim that one is working in Darwin's footsteps, or one is revising and correcting Darwin. Basically, Darwin did not do very much to our conception of species apart from make it more widely acceptable for naturalists to treat them as temporary things that grade into one another, something that was known or accepted well before him. If there was a crucial figure in our understanding of species today, it is not Darwin, but Mendel.
Mendelian genetics began around 1900 and, by 1904, a well known biologist, Edward Bagnell Poulton, had written an influential essay entitled 'What is a species?' The new genetics immediately had people trying to find genetic definitions: pure lines, gene groups etc, were all tried out and rejected. Thus was born what we now call the species problem. Darwin did not have a species problem, he had a species question: How do species originate? Mendel caused us to start producing the large number of species definitions - 26 at last count - that we now have.
In thinking about species we are dealing with the heart of science in many ways. Even now biologists sporadically tear into that concept and the variety of definitions in play. I don't expect that this will change any time soon.
