Darwin and the Nature of Species (review)

Despite the title of Charles Darwin's most celebrated work, he never clearly defined species. As a result, the question of what (if anything) Darwin really thought species were has been the subject of much discussion. By carefully examining Darwin's writings, both published and unpublished, David N. Stamos believes it is possible to end these debates by reconstructing Darwin's definition of species and his reasons for not stating it explicitly. In doing so, Stamos hopes to resolve a historical debate and provide some much needed clarity for contemporary biologists and philosophers of biology who are still debating the definition of species.

Stamos begins with the history of ideas about Darwin's species concept. He argues that those who have studied the problem fall into two groups: some argue that Darwin thought species do not exist but are merely convenient labels; others claim both that Darwin thought species were real and that his view was a precursor of whichever current view each writer maintains. Stamos falls into the latter group but argues that while previous writers have been mistaken about Darwin's species concept, he now understands it correctly. According to Stamos, Darwin's tacit definition rejected the assumption that each species possessed an unchanging "essence" (the common view among his contemporaries), since such a view created a sharp distinction between species and varieties (a fatal objection to Darwin's claim that varieties were no more than "incipient species"). Darwin also, at least for most of his life, rejected a "vertical" species concept, that is, the idea that species are like individuals: they are born, they exist during a period of time, and die. He therefore would not have been a cladist (a practitioner of the currently dominant form of classification, which classifies according to the order in which evolutionary lineages branched off from one another). Instead, Darwin was committed to a "horizontal" conception of species, seeing them as groups that exist and interact with each other at one specific time. Darwin, however, did not attribute much importance to "sterility, fertility or niches" (103), which are generally considered central to most modern "horizontal" definitions.

So, what, in Stamos's view, was Darwin's conception of species? They were effectively defined by their common adaptations, these being the result of natural selection, which Darwin regarded as a vera causa and thus, in effect, a natural law. In choosing to explain "both the existence of adaptation and the reality of species" in this way, "Darwin brought evolutionary biology (and ultimately the rest of biology) into the framework of the natural sciences, which were based on laws" (103). Stamos suspects, however, that Darwin never spelled out his definition because, despite his success in persuading his contemporaries that evolution had occurred, his central mechanism of natural selection was much less accepted. Since natural selection created species, there could be no definition of species until natural selection was more widely espoused.

Stamos's argument is generally sophisticated, subtle, and well supported. I suspect it will prove less persuasive than he hopes, however, because of the way he argues it. Throughout the book, Stamos shifts from historical to philosophical argument and back again in a way that will make many of his readers uneasy. For example, Stamos argues that "had Darwin known about polyploidy, I think it is safe to say he would have thought the cladistic interpretation of it pure nonsense" (74). This is the kind of claim that will please no one. Historians may object that what Darwin might have thought had he known what he could not possibly have known is not a topic that is amenable to historical analysis. Similarly, both philosophers and biologists might complain that invoking the authority of a long-dead naturalist to bolster an argument about the interpretation of current biology is, at best, redundant. If, as Stamos repeatedly argues, the concept of species he offers is correct, or at the very least superior to the alternatives...