The Origin was published when Darwin was 50. He lived from 1809 to 1882, in other words although he was sick for much of his adult life, he had 20 or more years of mature life before the Origin and the same amount after the Origin. One thing is true above all about Charles Darwin. He was a compulsive worker. No one had imbibed the Protestant work ethic more than he. With time out for illness, he worked steadily every day, including Sundays. And it showed, because a stream of works issued forth from his pen, from early studies of geology to late studies of earthworms.

Nothing could equal the Origin, but there is a natural and perpetual fascination with the other works. Some are fairly easy to assess. Probably the second most important Darwin book is the one on our own species, the Descent of Man [3], published some 12 years after the Origin. It is a somewhat odd work, because more than half is in fact not on the main topic, but rather on the secondary mechanism of sexual selection (there is a reason for this, namely that Darwin needed a mechanism to counter Alfred Russel Wallace's [1870] [4] objections to a natural origin for humankind, and sexual selection filled this role). It is also a work that draws far more heavily than the Origin, on the work of others, and that in respects is much more one of opinion and value judgment than the Origin (there is, for instance, lots of material on the natural superiority of Europeans). My sense is that today we rate the Descent somewhat higher than we would have done 50 years ago, at the 100th anniversary of the Origin. Sexual selection today is given a much bigger role than back then, and Darwin's detailed discussions of the evolution of morality have attracted much (positive) attention. I have just extracted them for a collection I am publishing on Darwin's importance for philosophy (Ruse [2009] [5]).

Among those of us who work on Darwin, my suspicion is that the favorite—perhaps even giving the Origin itself a run for its money—is the travel book, Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. (1839) [6], published in a popular version and better known as the Voyage of the Beagle (1845) [7]. Much of this is based on Darwin's diaries and his letters home to his family and it surely is a terrific read. We follow a bright and educated young man, literally on a voyage of discovery, both at sea and (for most of the time) on land—just as well the latter, since Darwin suffered dreadfully from seasickness. We stay with him as he discovered exotic flora and fauna in the Southern Hemisphere, as he uncovered the mysteries of the past recorded in the fossil record, and as he learns so much more about his fellow humans, be they slave owners or gauchos or (what he regarded as) “savages” down at the bottom of South America, in Tierra del Fuego. I love the book and I suspect many others do too (it is still available in English in about 10 different editions).

The volumes on geology—The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836 (1842) [8], Geological observations on the volcanic islands visited during the voyage of H.M.S. Beagle, together with some brief notices of the geology of Australia and the Cape of Good Hope. Being the second part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836 (1844) [9], Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836 (1846) [10]—seem me as more dutiful than exciting, although, of course, they do incorporate Darwin's brilliant hypothesis about the formation of coral reefs. But even this, that the reefs are formed by the sinking of land as the coral grows upwards, is part and parcel of a Lyellian approach to the rocks (for all that, Darwin's hypothesis was correcting an earlier hypothesis of Lyell himself), and ultimately what Thomas Kuhn would have called “normal” rather than “revolutionary” science. The volumes of barnacle taxonomy—A Monograph of the Sub-Class Cirripedia, with Figures of all the Species. The Lepadidae; or Pedunculated Cirripedes (1851) [11], A Monograph of the Fossil Lepadidae; or, Pedunculated Cirripedes of Great Britain (1851) [12], A Monograph of the Sub-Class Cirripedia, with Figures of all the Species. The Balanidge (or Sessile Cirripedes); the Verrucidae, and C. (1854) [13], A Monograph of the Fossil Balanidae and Verrucidae of Great Britain (1854) [14]—are detailed and obviously something for the professional in the field, rather than for general reading. Tradition praises the quality of the work, although more recent assessments have been highly critical. It is not yet sorted out whether Darwin provided a foundation on which others could build or if he simply went entirely astray and his work can be and should be ignored (Newman [1993] [15]).

After the Origin, I find the big two-volume work on Variation of Animals and Plants under Domestication (1868) [16] to be more worthy than exciting, filling out (as promised) the specific claims made (especially in the first chapter) in the Origin. I suspect that Darwin felt the same way, because despite promises, he certainly did not spend the rest of his life writing books based on each and all of the individual chapters of the Origin. The Expression of the Emotions in Man and Animals (1872) [17] likewise does not do much for me, being basically material left on the cutting floor after Darwin had finished the Descent of Man. I will say that one book I like very much is the little work that comes next after the Origin: On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects (1862) [18]. Darwin himself told his publisher that he regarded it as an example of how to do biology within the paradigm (as we would say) of the Origin. It is to show how thinking about adaptation can be done now. Something I find particularly fascinating is the way in which Darwin wrestles with adaptation, showing that descent with modification often means that things are not done as efficiently as they might if one could sit down and start from scratch whenever faced with a problem or demand. One has to work with what one has and this reflects in the finished product. The English would speak of this as being designed in the “Heath Robinson” mode, after a humorist who used to build fantastical machines, way over-complex, to do simple jobs.

So this then brings us to the books of Darwin's old age. Most of these strike me as fairly straightforward, in the sense that they represent projects that an old naturalist found fascinating—The Movements and Habits of Climbing Plants (1875) [19], Insectivorous Plants (1875) [20], The Power of Movement in Plants (1880) [21], and The Formation of Vegetable Mould, Through the Action of Worms, with Observations on their Habits (1881) [22]—that he could work on in a fairly low-tech sort of way, using simple tools and the time of himself and his gardeners. I would certainly not condemn them for being what they are, although equally I would not want to pretend that they are more than they are. In a sense, as many knew, the world of science had started to pass Darwin by in the 1870s—he was not up on the latest laboratory techniques or methods of experimentation. He was coasting down after a long life of effort (I have argued that in respects Darwin was out of date by the time he published the Origin—he was responding rather to the problems of the 1830s—but there I would certainly defend the proposition that it was the world that was out of kilter not Darwin. By the 1870s, I think the story is the other way around. I am not making this claim as the thin end of a very large wedge to belittle the achievements of the Origin).

2.1 Cross- and Self-Fertilization

Well, we can certainly say that Darwin is not wrong in his estimation of much of the book. It is grindingly dull! Page after page of results of experiments. One prays for the coming of the Internet, when the empirical information can be put on the net, for those who want to look at it, and put on the net, for the avoidance by those who do not want to look at it. Having said this, and therefore recognizing that line by line this is not a work for the ages, what about the work in perspective, as it were? Here it does become more interesting. It is part of an overall programme, one which indeed starts back even before Darwin discovered natural selection, just about at the time when he became an evolutionist. It seems to have been something at least in part spurred by reading his grandfather's, Erasmus Darwin's, evolution-mentioning work Zoonomia [26]. It is part of the quest for the understanding of sexuality. Why do we have two sexes? What is the biological reason for this, or since Darwin never asks a question without wanting to know the purpose—the final cause as Aristotle (and Darwin often) would say—what is sex for? Why go to the bother of having sex, when it would be so much simpler just to reproduce asexually?

The answer seems to be that somehow organisms need shaking up. If they just continue along the same path, without stresses and strains and changes, they seem to run out of steam. Hence, they need new conditions just to tone them up as it were, and more particularly they need sex to bring together two lines and shake things up internally. But it cannot be just two lines, indifferently. It has to be two lines coming in from different circumstances. Coming from the same background cuts the worth of the mating.

This means that as Darwin says in his Autobiography, we have a two-part project. In the first place, we need proof that organisms have adaptations for sexuality, and moreover for making sure that they do not cheat and fertilize themselves. I presume that humans are not particularly interesting here, for it is not that easy to see how humans could fertilize themselves. Plants, however, are more interesting and pertinent, because often they carry both male and females organs—they are hermaphrodites—and thus have the potential for self-fertilization. It was the task of the little book on orchids to show that nevertheless plants have adaptations making self-fertilization difficult if not impossible. This is a very Darwinian project, because natural selection would have been at work here, as Darwin shows in abundance.

In the second place, we need proof that self-fertilization is indeed bad for you, and this is the job of The Effects of Cross- and Self-Fertilisation in the Vegetable Kingdom, a job that Darwin prides himself on having done well and successfully.

“The first and most important of the conclusions, which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious. This is shown by the difference in height, weight, constitutional vigour, and fertility of the offspring from crossed- and self-fertilised flowers, and in the number of seeds produced by the parent-plants. With respect to the second of these two propositions, namely, that self-fertilisation is generally injurious, we have abundant evidence. The structure of the flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc., renders the aid of insects almost indispensable for their fertilisation; and bearing in mind the prepotency of pollen from a distinct individual over that from the same individual, such plants will almost certainly have been crossed during many or all previous generations. So it must be owing merely to the prepotency of foreign pollen, with cabbages and various other plants, the varieties of which almost invariably intercross when grown together. The same inference may be drawn still more surely with respect to those plants, such as Reseda and Eschscholtzia, which are sterile with their own pollen, but fertile with that from any other individual. These several plants must therefore have been crossed during a long series of previous generations, and the artificial crosses in my experiments cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation.

With respect to the first proposition, namely, that cross-fertilisation is generally beneficial, we likewise have excellent evidence. Plants of Ipomoea were intercrossed for nine successive generations; they were then again intercrossed, and at the same time crossed with a plant of a fresh stock, that is, one brought from another garden; and the offspring of this latter cross were to the intercrossed plants in height as 100 to 78, and in fertility as 100 to 51. An analogous experiment with Eschscholtzia gave a similar result, as far as fertility was concerned. In neither of these cases were any of the plants the product of self-fertilisation. Plants of Dianthus were self-fertilised for three generations, and this no doubt was injurious; but when these plants were fertilised by a fresh stock and by intercrossed plants of the same stock, there was a great difference in fertility between the two sets of seedlings, and some difference in their height. Petunia offers a nearly parallel case” (Darwin [1876] [23]: 436–437).

Note, as insisted, the cross-fertilization only works effectively if the organisms have been brought up in different circumstances. Presumably Darwin had use and disuse in mind as important operative factors here, since this was always a mechanism he endorsed and as is well known in the light of criticisms made of the arguments of the Origin (for instance about heredity), he grew to rely more on it rather than less as he grew older.

“It is obvious that the exposure of two sets of plants during several generations to different conditions can lead to no beneficial results, as far as crossing is concerned, unless their sexual elements are thus affected. That every organism is acted on to a certain extent by a change in its environment, will not, I presume, be disputed. It is hardly necessary to advance evidence on this head; we can perceive the difference between individual plants of the same species which have grown in somewhat more shady or sunny, dry or damp places. Plants, which have been propagated for some generations under different climates or at different seasons of the year, transmit different constitutions to their seedlings. Under such circumstances, the chemical constitution of their fluids and the nature of their tissues are often modified. Many other such facts could be adduced. In short, every alteration in the function of a part is probably connected with some corresponding, though often quite imperceptible change in structure or composition.

Whatever affects an organism in any way likewise tends to act on its sexual elements. We see this in the inheritance of newly acquired modifications, such as those from the increased use or disuse of a part, and even from mutilations if followed by disease. We have abundant evidence how susceptible the reproductive system is to changed conditions, in the many instances of animals rendered sterile by confinement; so that they will not unite, or if they unite do not produce offspring, though the confinement may be far from close; and of plants rendered sterile by cultivation. But hardly any cases afford more striking evidence how powerfully a change in the conditions of life acts on the sexual elements, than those already given, of plants which are completely self-sterile in one country, and when brought to another, yield, even in the first generation, a fair supply of self-fertilised seeds” (Darwin [1876] [23]: 446–447).

What is missing in all of this, obviously, is any reason why exactly it should be that organisms need shaking up. What is the point behind this? Why should organisms not simply keep going on in a straight line as it were, as obviously Darwin knew some asexual organisms do very happily for generation after generation? Those of us who know of the ideas of Herbert Spencer might think that something along the lines that he suggested might have appealed to Darwin, namely that it is all a matter of equilibrium lost and having to regain this equilibrium but at a higher level after disruption. However, although Darwin may have been tempted, as always with Spencer he rather distrusted the metaphysics simulating real science.

“The fertilisation of one of the higher plants depends, in the first place, on the mutual action of the pollen-grains and the stigmatic secretion or tissues, and afterwards on the mutual action of the contents of the pollen-grains and ovules. Both actions, judging from the increased fertility of the parent-plants and from the increased powers of growth in the offspring, are favoured by some degree of differentiation in the elements which interact and unite so as to form a new being. Here we have some analogy with chemical affinity or attraction, which comes into play only between atoms or molecules of a different nature. As Professor Miller remarks: “Generally speaking, the greater the difference in the properties of two bodies, the more intense is their tendency to mutual chemical action… But between bodies of a similar character the tendency to unite is feeble”. This latter proposition accords well with the feeble effects of a plant's own pollen on the fertility of the mother-plant and on the growth of the offspring; and the former proposition accords well with the powerful influence in both ways of pollen from an individual which has been differentiated by exposure to changed conditions, or by so-called spontaneous variation. But the analogy fails when we turn to the negative or weak effects of pollen from one species on a distinct species; for although some substances which are extremely dissimilar, for instance, carbon and chlorine, have a very feeble affinity for each other, yet it cannot be said that the weakness of the affinity depends in such cases on the extent to which the substances differ. It is not known why a certain amount of differentiation is necessary or favourable for the chemical affinity or union of two substances, any more than for the fertilisation or union of two organisms.

Mr Herbert Spencer has discussed this whole subject at great length, and after stating that all the forces throughout nature tend towards an equilibrium, remarks, “that the need of this union of sperm-cell and germ-cell is the need for overthrowing this equilibrium and re-establishing active molecular change in the detached germ—a result which is probably effected by mixing the slightly-different physiological units of slightly-different individuals”. But we must not allow this highly generalised view, or the analogy of chemical affinity, to conceal from us our ignorance. We do not know what is the nature or degree of the differentiation in the sexual elements, which is favourable for union, and what is injurious for union, as in the case of distinct species. We cannot say why the individuals of certain species profit greatly, and others very little by being crossed. There are some few species, which have been self-fertilised for a vast number of generations, and yet are vigorous enough to compete successfully with a host of surrounding plants. We can form no conception why the advantage from a cross is sometimes directed exclusively to the vegetative system, and sometimes to the reproductive system, but commonly to both. It is equally inconceivable why some individuals of the same species should be sterile, whilst others are fully fertile with their own pollen; why a change of climate should either lessen or increase the sterility of self-sterile species; and why the individuals of some species should be even more fertile with pollen from a distinct species than with their own pollen. And so it is with many other facts, which are so obscure that we stand in awe before the mystery of life” (Darwin [1876] [23]: 456–458).

That basically is it. In the end Darwin has to profess ignorance. Let us say charitably that in the total ignorance of a theory of heredity, it was hardly a case of failure that he could not puzzle out sexuality. Many feel that we have failed to do so even to this day. But we can say that here was an area where Darwin only managed to give part of the solution. He could show that cross-fertilization was important, but he could not show why.

In these days of social constructivism, some readers will have remembered that Darwin married a first cousin and wonder if his work on cross-fertilization was at all connected to this. Without being definitive, I rather doubt it, if only because Darwin was worrying about sexuality before he proposed to and married Emma Wedgwood. However, Darwin does raise the issue in this book under discussion, if obliquely.

“With respect to mankind, my son George has endeavoured to discover by a statistical investigation whether the marriages of first cousins are at all injurious, although this is a degree of relationship which would not be objected to in our domestic animals; and he has come to the conclusion from his own researches and those of Dr Mitchell that the evidence as to any evil thus caused is conflicting, but on the whole points to its being very small. From the facts given in this volume, we may infer that with mankind, the marriages of nearly related persons, some of whose parents and ancestors had lived under very different conditions, would be much less injurious than that of persons who had always lived in the same place and followed the same habits of life. Nor can I see reason to doubt that the widely different habits of life of men and women in civilised nations, especially amongst the upper classes, would tend to counterbalance any evil from marriages between healthy and somewhat closely related persons” (Darwin [1876] [23]: 460–461).

One hears the voice of the upper-class Charles Darwin who had spent his youth clambering over the rocks of South America while Emma stayed at home in rural Staffordshire playing the piano and visiting the deserving poor.