Darwin's Origin of species, sixth edition (1872)
An introduction by Gordon Chancellor
Darwin started work on this edition in June 1871 once the fuss over Descent of Man, published at the start of that year, had started to die down. He finished writing the following January, having decided that responding to the criticisms of Mivart (1872) required a whole new chapter. In February 1872 Murray printed 3,000 copies of this smaller, cheaper edition but because Darwin only ever made tiny corrections (in 1876) we may say that every copy printed for the rest of his life was the sixth edition, totalling c. 14,000 copies. Murray was aiming for the new market of readers who had benefitted from the 1870 Education Act. They certainly bought the book but may have been discouraged by the small type face from actually reading it. Finally, the whole book was reset in 1891.
Although the book appears to differ considerably from previous editions this is largely because of its smaller type and cheaper format. 'On' is dropped from the title, so Darwin was finally declaring that his was the explanation for the origin of species.
This edition actually had less changes from its previous edition than did the fifth. 63 sentences were dropped, but 571 were added and 1,669 altered and the word count shot up by 17,000. The new chapter seven has also tended to make it seem more different than it really was. It did however have a glossary added (by W. S. Dallas) which has been used in many subsequent reprints – this being the edition most commonly reprinted once the book came out of copyright. This edition was the first in which Darwin used the word 'evolution', although he had used it in Variation under domestication (1868, vol. 2, p.60) with a different meaning.
There are only minor changes to the first three chapters but in chapter four Darwin makes the major changes which resulted in the need for a new seventh chapter to deal with 'miscellaneous objections to the theory of natural selection'. In order to make comparison between the sixth and fifth editions we refer here to chapters after six as 'former seven', 'former eight' etc. The only significant addition to chapter four is on the vast numbers of eggs and seeds which are fortuitously destroyed and therefore have no bearing on the course of evolution. Darwin makes clear however that it is inevitable that this will happen and that it is no objection to his theory, as natural selection will apply to the few survivors and that is what matters. The other significant change to chapter four is on the convergence of specific forms.
There are a few changes to chapter five but in chapter six there are amendments on the woodpecker of La Plata and on the modification of the eye. Darwin adds a long paragraph on the role in evolution of heterochrony, that is the effects of changes in the rates of development such as retardation of sexual maturity. We now know that such changes, which have certainly had major effects, may be due to tiny changes in the timing of the switching on and off of certain genes. There are amendments on electric fish, eyes of cephalopods, hair claspers of acarid mites, rattlesnake rattles and the imperfection of the human eye. Darwin makes a fascinating addition concerning the Creator in the following sentence:
They believe that many structures have been created for the sake of beauty, to delight man or the Creator (but this latter point is beyond the scope of scientific discussion), or for the sake of mere variety, a view already discussed. (p. 162)
[The fifth edition read: “They believe that many structures have been created for beauty in the eyes of man, or, as already discussed, for the sake of mere variety.”]
Darwin's own summary of the new chapter seven is from his list of additions and corrections:
The first part of this new chapter consists of portions, in a much modified state, taken from chapter four of the former editions. The latter and larger part is new, and relates chiefly to the supposed incompetency of natural selection to account for the incipient stages of useful structures. There is also a discussion on the causes which prevent in many cases the acquisition through natural selection of useful structures. Lastly, reasons are given for disbelieving in great and sudden modifications. Gradations of character, often accompanied by changes of function, are likewise here incidentally considered.
One could add that the shorter first part reads like a rag bag of recent research findings and at best adds little and at worst rather detracts from the coherence of Darwin's great book. The second part, however, is a tour de force and although – as Darwin himself hints – it may be just too long, it is a major addition to the Origin. It is essentially a response to Mivart's Genesis of species and it is fascinating to see how Darwin demolishes all of Mivart's arguments, which are mostly about animals but also orchids and climbing plants. The whole episode backfired on Mivart because Darwin tells us that having to combat Mivart's arguments convinced him totally of the truth of his theory.
The former chapter seven contains some changes to the discussion of cuckoo instincts and to the discussion of 'community selection'. There are amendments on hybrid sterility in former chapter eight and a few significant changes in former chapters nine and ten of which perhaps the most interesting is the following response to the physicist Lord Kelvin's views on Earth history:
It is, however, probable, as Sir William Thompson insists, that the world at a very early period was subjected to more rapid and violent changes in its physical conditions than those now occurring; and such changes would have tended to induce changes at a corresponding rate in the organisms which then existed. (sixth edition p. 286; not fifth edition p. 354 as stated by Costa 2009, p. 493)
Darwin refers to this objection again in his final chapter (see below). There are minor changes to former chapter eleven on mud on birds' feet and former chapter twelve, notably the 'wonderful case' of the freshwater fish Galaxias from the Antarctic region.
In the former chapter thirteen there are several new large paragraphs on subjects such as marsupial feet, serial homologies – today the focus of much research in the field of evolutionary developmental biology ('evo-devo'); asexual reproduction in mosquitos and the tricky issue of why an organ which has fallen out of use will continue to wither.
Finally, in former chapter fourteen there is a new long paragraph on the sterility of hybrids and a more philosophical response to Lord Kelvin's objection:
With respect to the lapse of time not having been sufficient since our planet was consolidated for the assumed amount of organic change, and this objection, as urged by Sir William Thompson, is probably one of the gravest as yet advanced, I can only say, firstly, that we do not know at what rate species change as measured by years, and secondly, that many philosophers are not as yet willing to admit that we know enough of the constitution of the universe and of the interior of our globe to speculate with safety on its past duration. (p. 409)
On page 421 a rather exasperated Darwin repeats yet again his statement from the first edition that he never claimed natural selection as the sole cause of evolution, and as Hoquet (2013, p.164) has shown claims that Darwin somehow became more 'Lamarckian' as he got older do not stand up to scrutiny.
Finally, Darwin adds a substantial paragraph which can stand as an epitome for this whole introduction:
As a record of a former state of things, I have retained in the foregoing paragraphs, and elsewhere, several sentences which imply that naturalists believe in the separate creation of each species; and I have been much censured for having thus expressed myself. But undoubtedly this was the general belief when the first edition of the present work appeared. I formerly spoke to very many naturalists on the subject of evolution, and never once met with any sympathetic agreement. It is probable that some did then believe in evolution, but they were either silent, or expressed themselves so ambiguously that it was not easy to understand their meaning. Now things are wholly changed, and almost every naturalist admits the great principle of evolution. There are, however, some who still think that species have suddenly given birth, through quite unexplained means, to new and totally different forms: but, as I have attempted to show, weighty evidence can be opposed to the admission of great and abrupt modifications. Under a scientific point of view, and as leading to further investigation, but little advantage is gained by believing that new forms are suddenly developed in an inexplicable manner from old and widely different forms, over the old belief in the creation of species from the dust of the earth. (p. 424)