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Darwin Correspondence Project

Species and varieties

Origin_1859_Title page_c.jpg

Origin of species (1859) title page
Origin of species (1859) title page

On the origin of species by means of natural selection …so begins the title of Darwin’s most famous book, and the reader would rightly assume that such a thing as ‘species’ must therefore exist and be subject to description. But the title continues, …or the preservation of favoured races in the struggle for life. This is more ambiguous, especially to the modern reader, for whom race carries a different and highly charged meaning. In the context of natural history, when Darwin used the term here, he simply meant ‘variety’, as in ‘a fast-growing race of wheat’. The question, then, is whether Darwin thought that species were ‘real’ or whether boundaries among races or varieties were so fluid that clearly defined species didn’t exist. The answer to the question is … both!


‘Undiscoverable essence’

In one of his reading notebooks, under the lists of books he wanted to read (DAR 119: 2v), Darwin scribbled a reminder to himself in 1838 to ‘read Aristotle to see if any of my views are ancient’. He never got around to reading Aristotle beyond a few extracts, until shortly before his death, when William Ogle sent his translation, Aristotle on the parts of animals (Ogle trans. 1882). Darwin would have found that Aristotle seemed to deny the existence of fixed beings when he wrote, ‘For nature passes from lifeless objects to animals in such unbroken sequence, interposing between them beings which live and yet are not animals, that scarcely any difference seems to exist between two neighbouring groups owing to their close proximity’ (ibid., p. 104). Contrast this with Aristotelian essentialism, the idea that certain attributes are essential to the characterisation of things, and you have, in a nutshell, the two sides of a debate about the nature of living things that continued from antiquity.

The notion of an essence or essential characters that define or delimit organisms was central to Linnaeus’s great taxonomic project, Systema naturæ, and to most subsequent attempts at systematising organisms until Darwin published his own taxonomic works between 1851 and 1854. Linnaeus ordered the world according to an artificial system; that is, he chose a specific group of structural features by which he ordered organisms, without regard for environmental or other criteria. He was challenged by others who searched for a more ‘natural’ taxonomy that would exemplify the actual relationships of species, genera, orders, and classes. The notion of a defining characteristic seems straightforward only until it is put into practice when faced with a group of organisms. Choosing characters on which to base inclusion in a group is fraught with difficulty. In taxonomy, the characteristics that formed the basis for these choices were known as diagnostic features, but the trick was choosing which features were essential; that is, which features could be used to diagnose the place of an individual organism.

Darwin himself did not set out to be a taxonomist, but in trying to understand some of the anatomical anomalies of a tiny parasitic barnacle, he was eventually led to revise the taxonomy of the whole group, including extinct species. His approach to taxonomy was similar to those who sought to determine actual relationships; that is, he focused on how one species or variety related to other similar ones. He went further, however, not just trying to determine similarities in structure and habits but trying to discover relationships based on descent from earlier forms. He always tried to make descriptions after having seen several individuals, having noted after years of observation just how much variability often existed within a species. The features he focused on were not simply morphological, but were those clearly related to each animal’s place in the environment: adaptive features. This taxonomy was underpinned by the idea that the relations in features reflected a real genealogical relationship over time. In the four volumes on the sub-class Cirripedia, Darwin maintained the distinction between species and varieties and even complained while working on his descriptions, ‘Literally not one species is properly defined: not one naturalist has ever taken the trouble to open the shell of any species to describe it scientifically, & yet all the genera have 1/2 a dozen synonyms’ (letter to H. E. Strickland, [4 February 1849]).

In the conclusion to Origin, Darwin stated his belief about systematics informed by an evolutionary point of view, noting, ‘Systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be in essence a species.’ He continued, regarding species and varieties, ‘Hereafter we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is, that the latter are known, or believed, to be connected at the present day by intermediate gradations, whereas species were formerly thus connected. Hence, without quite rejecting the consideration of the present existence of intermediate gradations between any two forms, we shall be led to weigh more carefully and to value higher the actual amount of difference between them. …we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species’ (Origin, p. 485).

‘To define the undefinable’

Darwin developed his theory of evolution by natural selection over many years and gave a lot of thought to definitions of species both implicit and explicit in the work of contemporary naturalists. In a letter to his friend Joseph Hooker, he wrote, ‘It is really laughable to see what different ideas are prominent in various naturalists minds, when they speak of “species” in some resemblance is everything & descent of little weight—in some resemblance seems to go for nothing & Creation the reigning idea—in some descent the key—in some sterility an unfailing test, with others not worth a farthing. It all comes, I believe, from trying to define the undefinable’ (letter to  J. D. Hooker, 24 December [1856]). The idea that sterility was a test of species was firmly held by Thomas Huxley, who argued that until Darwin could demonstrate the possibility of developing from a particular stock, by selective breeding, two forms, which should either be unable to cross one with another, or whose cross-bred offspring should be infertile with one another, the theory of evolution by natural selection would not be ‘beyond the reach of all possible assault’ (T. H. Huxley 1863a, p. 147).

In Origin p. 272, Darwin had argued that the sterility of interspecific hybrids was not a special endowment but was gradually acquired through divergent modifications in the reproductive systems of the forms that were crossed; in other words, as incipient new species diverged from each other while undergoing various structural changes, some changes to reproductive organs would eventually result in two related species no longer being able either to breed, or to produce fertile offspring. Huxley’s challenge to create two forms infertile with one another would have been hard to achieve in practice, given that Darwin’s theory required that small variations over very long periods of time were required to effect change. Darwin began to look at sterility from a different perspective. In May 1860, he noticed differences in the flowers of cowslips, some plants having long stamens (male organs) and short pistils (female organs), others with short stamens and long pistils; he also noted differences in the size of the pollen in the two forms. Over the next decades, Darwin carried out numerous experiments with many plant species whose flowers had two or three different forms and published five articles and eventually a book, The different forms of flowers on plants of the same species (1877) What Darwin discovered was that many, but by no means all, different sexual forms were often sterile if crossed with the same form. Sometimes all different forms of a species were self-sterile, sometimes one form was self-sterile, while another was fertile with its own pollen. There seemed to be no universal law about sterility, even in members of the same species.

Throughout the 1860s, Darwin vacillated about whether sterility could be ‘selected’. In 1862, he told Hooker, ‘I am now strongly inclined to believe that sterility is at first a selected quality to keep incipient species distinct’ (letter to J. D. Hooker, 12 [December 1862]). In 1866, Darwin compared the sterility of hybrids, or the offspring of hybrids, with the sterility of same-form unions among dimorphic and trimorphic plants; he evidently still thought the latter might be caused by natural selection (Origin 4th ed., pp. 323–6). However, by 1868, in The variation of animals and plants under domestication, Darwin argued that degrees of lessened fertility could not have been slowly accumulated by means of natural selection because no advantage would accrue from fewer offspring resulting from crosses with other varieties, and therefore lessened fertility would not be 'selected'. (Variation 2: 186–7). One of the reasons that Darwin’s views on the subject crystallised at this time was his discussion of the issue with Alfred Wallace in the spring of 1868. Wallace had sent a concise elaboration of how the sterility of hybrids might be produced by natural selection (letter from A. R. Wallace, 1 March 1868). Darwin turned to his son George, a mathematician, who stated the problem as a mathematical equation, finding that natural selection could not increase sterility. Wallace was not convinced by the objections and the exchange continued briefly until he decided to ‘say no more but leave the problem as insoluble’ (letter from A. R. Wallace, 8 [April] 1868).

Ultimately, Darwin’s view was practical and flexible. Species could be defined in relation to each other, and, over time, these relations could change in response to the continually changing environment. Varieties might be potential new species, and present species could once have been varieties. The relations among species were not derived from some idealistic notion of essences, but from actual genealogical relationships over time.