Most historians of evolutionary biology have contended that Darwin did not believe that species were real. Instead, they claim, he believed species were arbitrarily delimited from each other, and the species was nothing more than a more distinct variety. Thus, according to Mayr, Darwin did not attempt to solve the problem of speciation because he did not and--because of his species concept, could not--appreciate that there was a problem to be solved. Since he did not consider the species a distinct natural unit, it was only natural that he did not see the need to explain how species multiply. [Kottler 1978]
Darwin's early evolutionary views of species have been well studied, in particular by Kottler whose comment above shows the "Standard View" with regards to Darwin's conceptual development. In this post, I'll look at the Transmutation Notebooks Darwin kept from his realisation that species evolved, through to his beginning to work on what he came to call his "big species book". These offer us a glimpse into the issues and thought processes that led to Darwin's evolutionary theory (or, rather, theories, for there are several of them).
While he was en route on the Beagle, he received Charles Lyell's Principles of Geology volume 2 (1831), which was almost entirely devoted to a discussion of the evolutionary views of Lamarck.
Lyell's discussion was extensive. Oddly, until that point, Lamarck had little exposure in English speaking countries, but in the period of the 1830s, evolution was "in the air" among political radicals. In France, Geoffroy St Hilaire, and his discipline John Grant in Scotland, were pushing a variety of Lamarck's evolutionary views, and so evolution was tainted by its political radical affiliations. So perhaps Lyell thought it was time to knock this view on the head. Whatever his motivation, he managed to make evolution a widely discussed idea. He also inspired Darwin to think about what species were.
Lamarck had denied that species were anything more than the product of human classification. Like his mentor Buffon, he thought that in the real biological world, there were only individual organisms. Lyell observed that if species were real, then they were fixed, within limits. Lamarck agreed with this view. What they disputed (or would have, had Lamarck not died a decade or so earlier) was whether species were real things. Lamarck had been strongly influenced by John Locke, as had many 18th Century French naturalists, and so thought that species were conventions, made for ease of use. Lyell, more influenced by idealist philosophy, rejected this, and being pious, held that the view that species were as God had made them, and could only vary so far. Lyell ended up stating, rather emphatically
The intermixture of distinct species is guarded against by the aversion of the individuals composing them to sexual union, or by the sterility of the mule offspring.
...species have a real existence in nature; and that each was endowed, at the time of its creation, with the attributes and organization by which it is now distinguished [1st edition, vol2 p. 65; 5th edition, 1837, vol I, 518. Darwin annotated this edition, not the first, when he began to fill up his notebooks, according to Kottler.]
As Kottler noted:
For Lyell, 'whether species have a real and permanent existence in nature ?' (vol. 1, 481; my italics) was one question.
Darwin began by accepting Lyell's view that species were kept distinct by "mutual aversion and cross-sterility", but rejecting, of course, the notion that species had to be fixed. So he had to maintain the view that species were real things (causally), which was one tine on Lyell's fork, but reject Lamarck's view that transmutation made species arbitrary. How did he do this?
It is daily happening, that naturalists describe animals as species ... There is only two ways [sic] of proving to them it is not; one where they can [be] proved descendant [Kottler interpolates: descent from common parents], which of course most rare, or when placed together they will breed. [Notebook B122]
As species is real thing with respect to contemporaries – fertility must settle it [C152]
If they [systematists – JSW] give up infertility in largest sense as test of species – they must deny species which is absurd. [E24].
So initially, Darwin thought that a species was defined by interfertility within, and infertility between, species, and that they were real. But over time, he came to downplay the role of interfertility. While fertility was one thing that could make a temporary species, it was not the sine qua non - in fact selection was. In effect Darwin came to think species were ecological, in modern terms.
In his letters, he was more forthcoming:
It has long appeared to me, that the root of the difficulty in settling such questions as yours, – whether the number of species &c &c should enter as an element in settling the value of existence of a group – lies in our ignorance of what we are searching after in our natural classifications. – Linnaeus confesses profound ignorance. – Most authors say it is an endeavour to discover the laws according to which the Creator has willed to produce organized beings – But what empty high-sounding sentences these are – it does not mean order in time of creation, nor propinquity to any one type, as man. – in fact it means just nothing – According to my opinion, (which I give everyone leave to hoot at, like I should have, six years since, hooted at them, for holding like views) classification consists in grouping beings according to their actual relationship, ie, their consanguinuity, or descent from common stocks … [Letter to G. R. Woodhouse, 26 July 1843]
I was so struck with the distributions of Galapagos organisms &c &c & with the character of the American fossil mammifers, &c &c that I determined to collect blindly every sort of fact, which cd bear any way on what are species … At last gleams of light have come, & I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable. [Letter to Hooker, 11 January 1844]
I knew, of course, of the Cuvierian view of Classification, but I think that most naturalists look for something further, & search for ‘the natural system’, – ‘for the plan on which the Creator has worked’ &c &c. – It is this further element which I believe to be simply genealogical. [Darwin to Huxley 3 October 1853]
So by the 1850s he was clear - classification rests on genealogy, and not on interfertility or similarity:
You speak of species not having any material base to rest on; but is this any greater hardship than deciding what deserves to be called a variety & be designated by a greek letter. When I was at systematic work, I know I longed to have no other difficulty (great enough) than deciding whether the form was distinct enough to deserve a name; & not to be haunted with undefined & unanswerable question whether it was a true species. What a jump it is from a well marked variety, produced by natural cause, to a species produced by the separate act of the Hand of God. But I am running on foolishly. – By the way I met the other day Phillips, the Palaeontologist, & he asked me “how do you define a species?” – I answered “I cannot” Whereupon he said “at last I have found out the only true definition, – ‘any form which has ever had a specific name’! … [letter to Asa Gray, 29 November 1857]
But he did make one point - species definitions did not seem to affect the conventions of taxonomic work:
... in my own work, I have not felt conscious that disbelieving in the permanence of species has made much difference one way or the other; in some few cases (if publishing avowedly on doctrine on non-permanence) I shd. not have affixed names, & in some few cases shd. have affixed names to remarkable varieties. Certainly I have felt it humiliating, discussing & doubting & examining over & over again, when in my own mind, the only doubt has been, whether the forms varied today or yesterday (to put a fine point on it, as Snagsby would say). After describing a set of forms, as distinct species, tearing up my M.S., & then making them one again (which has happened to me) I have gnashed my teeth, cursed species, & asked what sin I had committed to be so punished: But I must confess, that perhaps the same thing wd. have happened to me on any scheme of work—... [Darwin to Hooker, 25 September 1853]
I'm sorry for the lateness of this post. I started to organise my material on species definitions - not just for Darwin but for everybody from Aristotle to the modern day. It's currently at 133pp of single spaced A4, with commentaries on most of the excerpts done already. I'll link to it when it's done.
Thanks for that. Fascinating.
That Darwin guy sure was one smart cookie, wasn't he.