Charles Robert Darwin Research Paper

Although he developed an extracurricular interest in the natural sciences, the young Darwin was considered a mediocre student. At the age of 16 he was sent to Edinburgh to study medicine. He interacted with scientists there and furthered his interests in natural history, but he had no desire to become a physician and realized he would not need to earn a living. His father however insisted upon his having a respectable profession, and it was decided that he would prepare to become a clergyman. He therefore enrolled at Cambridge University on October 15, 1827, and remained there until 1831. His academic performance was respectable, though not outstanding. But his scientific interests were encouraged by the faculty, especially the botanist John Stevens Henslow (1796–1871).

Darwin was preparing for some scientific travel when he was invited to join a naval surveying expedition on HMS Beagle as unofficial naturalist and gentlemanly companion to the captain. The Beagle departed on December 27, 1831, circumnavigated the globe, and returned on October 2, 1836. This long period of field research laid the foundation for Darwin’s later career. Working in isolation no doubt forced him to think for himself, and he had much opportunity for reflecting on the many books he had with him, especially the Principles of Geology (1830–2) by Charles Lyell (1797–1875). Darwin became one of Lyell’s major supporters. While in South America he developed a theory of coral reefs which he verified later on during the voyage. It was his geological work that first made him famous. However he also studied zoology and botany and assembled a superb collection of specimens.

Darwin did not become an evolutionist until after his return to England, though he began to take the idea seriously toward the end of the voyage. Back in England he began to sort out his specimens and put them in the hands of specialists. Expert opinion, especially on the birds of the Galapagos Archipelago, convinced him (ca. early March, 1837) that evolution was a viable idea, and he set out to develop a theory. That included a search for a mechanism. His note-books indicate a vast amount of reading and thinking about many topics, including psychology.

Darwin’s discovery of natural selection occurred around September 28, 1838, when he read the Essay on the Principle of Population (Malthus 1826). Its author, who went by the name of Thomas Robert Malthus (1766–1834), was the first professor of economics. Darwin had already changed his conception of the natural economy to a competitive one but he had not made the crucial connection. The insight came from understanding what happens to human reproduction as resources get harder to come by with an increasing number of mouths to feed. It then became evident that some individuals would reproduce more than others. How much economics Darwin actually knew is questionable, but the general notion of individualistic competition was clearly available from the economics of his day. When, in the 1850s, Darwin developed his ideas about biodiversity he was to some extent helped by his reading of the French zoologist Henri Milne-Edwards (1800–85), who applied the idea of the division of labor to physiology and anatomy. Darwin realized that specialization would make for more efficient use of resources. Evolving populations would seize upon different places in the economy of nature (Eltonian niches, as they are now called).

Darwin married his cousin Emma Wedgwood on January 29, 1839; their first child was born on December 27 of that year. They set up house in London, but his health began to deteriorate and he suffered from mysterious ailments for the rest of his life. In 1842, the family relocated to a country residence near Down, south of London. Nonetheless Darwin was highly productive. A book on his voyage (Darwin 1839) was a popular as well as a scientific success. He edited the zoology of the voyage and, in addition to minor papers, published three volumes on his geo-logical research: (Darwin 1842, 1844, 1846). Development of his ideas on evolution continued, and he wrote a sketch of a book on it in 1842, and a draft in 1844.

In 1846, Darwin began some research on barnacles which led him to spend eight years revising the group and writing a vast taxonomic monograph (Darwin 1851, 1854) which in effect was his first book on evolution. He then proceeded to work up his materials on evolution. A large book entitled Natural Selection was in the process of composition when, on June 18, 1858, a manuscript from Alfred Russel Wallace (1823–1913) arrived in which the same basic mechanism was proposed. A joint paper was read before the Linnean Society on July 1, and on July 20, Darwin began to write an ‘abstract’ that ultimately became the Origin of Species (Darwin 1859). The first of six editions was published on November 24, 1859.

Darwin brought out a series of books that dealt with a broad range of evolutionary topics. These books all included results of his ongoing empirical research, much of it on the sexuality, physiology, and behavior of plants. A book on orchids (Darwin 1862) was followed by one on climbing plants that first appeared as a journal article (Darwin 1865). What had been a section of Natural Selection became The Variation of Animals and Plants Under Domestication (Darwin 1868). It was the second of what are considered Darwin’s ‘major’ evolutionary works. The third was The Descent of Man, and Selection in Relation to Sex (Darwin 1871). The Expression of the Emotions in Man and Animals (Darwin 1872) was followed by a series of botanical works (Darwin 1875, 1876, 1877a, 1877b, 1880). The last of his books dealt with earthworms, especially their behavior (Darwin 1881). Darwin died at his home at Down on April 19, 1882. He was buried at Westminster Abbey on April 26.

2. Darwin’s Scientific Contribution

Darwin brought about the most important revolution in the history of the human intellect. For that very reason it has taken a long time for his ideas to become assimilated and the process is far from complete. Although he was able to convince the scientific community that evolution had in fact taken place, he was far less persuasive with respect to natural selection, which only became the majority view towards the middle of the twentieth century. Furthermore, evolution as Darwin and his successors have come to understand it was something quite different from the ‘evolution’ of his predecessors and many of his contemporaries. This is true of both evolution and its mechanisms.

Darwin’s early geological research was crucial to his intellectual development. It gave him experience in thinking historically, dealing with real events that happened to concrete particular things. The fossil record, however, was still very poorly understood, and one of Darwin’s major contributions was showing how incomplete it was. It was biogeography, not paleontology, that convinced Darwin that evolution had occurred and provided his most compelling arguments for it. But that required thinking about groups of organisms not merely changing but also having common ancestors. Darwin’s predecessor Lamarck (1744–1829) had published a branching diagram, but his basic conception was that of multiple origins of simple organisms by spontaneous generation, and their descendants becoming increasingly complex as independent lineages. It was a more historical version of the traditional idea of a scala naturae, with organisms ranked as higher or lower according to how closely they resembled human beings.

When Darwin’s predecessors attempted to explain the transformation of species they invoked the kinds of causation that they were familiar with in other areas of experience. Supernatural influences played a diversity of roles in such thinking, often important ones. Change in lineages was largely thought of in terms of analogies with the development of embryos. A kind of ‘evolutionism’ was possible in which simple organisms had been created followed by a long succession of generations before the result that had been foreseen from the beginning—Man—was realized. Alternatively, the Creator had ordained laws of nature, such that matter would organize itself into increasingly complicated forms with Man, again, the preordained result. These unknown laws of nature were often conceptualized as similar to those that govern the formation of crystals.

In Darwin’s theory evolution was subject to the laws of nature, but there was no law or other cause that drove things in any particular direction. Organisms evolved the properties that they did because of historical contingencies, and Darwin’s critics often equated such contingency with ‘chance.’ Attempting to find explanations for evolution in terms of ‘law’ rather than ‘chance’ has been a recurrent theme in efforts to find alternatives to Darwinism.

The term ‘orthogenesis’ is often applied to notions of evolution occurring along predetermined lines, especially when there is some sort of analogy with the formation of an embryo. Such quasi-evolutionary ideas began to receive attention from social scientists before the publication of the Origin of Species (Darwin 1859). The stage theory of Auguste Comte (1798– 1857), with its theological, metaphysical, and positivistic levels, is one important example. Closer to what we moderns think of as evolution was the philosophy of Herbert Spencer (1820–1903). Spencer analogized both biological and socio–economic evolution with the embryology of Karl Ernst von Baer (1791–1876). In Spencer’s ‘synthetic philosophy,’ the increasing specialization of the parts in developing organisms was analogized with the specialization and diversification that occurs in society.

The ‘super-organism’ view of social and biological evolution has been firmly rejected by Darwin’s followers. However, a more limited form of ‘holism’ that affirms the reality and importance of supra- organismal groups has become increasingly popular in the philosophy of systematics. Ontologically, species and other taxonomic groups are now considered not to be classes but individuals in the broadest sense: particular, concrete things that have a definite position in space and time. Like individuals in the familiar sense of organisms, they can change indefinitely, they have no definitions, and there are no laws of nature for any one of them.

Species are now understood to be supra-organismal wholes that are held together by sex. Darwin however never had a satisfactory species concept. But he did emphasize the interaction between organisms and the reproductive competition among them as the fundamental cause of change. As he saw it, the com-petition for such resources as food is important only insofar as it affects reproductive success. The crucial role of sexual reproduction is underscored in Darwin’s theory of sexual selection, a major topic in The Descent of Man (Darwin 1871). The ‘secondary’ sexual characters, such as gaudy plumage in birds and the antlers of stags, were interpreted by Darwin as the result of purely reproductive competition, based upon monopolizing mates, and unrelated to survival or procreation pure and simple.

Misunderstandings about competition have always been a major impediment to the assimilation of Darwin’s theory. In economics, competition is thought of as a market condition that is antithetical to monopoly. The lay person, however, tends to confuse competition with other phenomena, such as physical combat, and to treat it as if it were the opposite of cooperation. For many, even teachers who grade on a curve, it is something antisocial, if not downright immoral. Of course political considerations have added to the confusion but it also reflects a deeper, metaphysical problem. The pre-Darwinian mentality has generally tended to assume that the world was created by God and that, in the words of Alexander Pope, ‘Whatever is, is right.’ Short of rejecting the basic premise, there are two popular alternatives. One is to endorse the conditions that exist. The other is to deny their existence. Various forms of Social Darwin-ism have opted for the former. More tender-minded social thinkers, from Prince Kropotkin to Stephen Jay Gould, have opted for the latter, portraying the natural economy as one in which competition is unimportant. Darwin of course rejected the basic premise. That perhaps bothered his contemporaries more than any other aspect of his theory. It allowed the phenomena of life to be explained in purely natural-science terms, without recourse to teleology in any but a trivial sense. Everything came into existence as a matter of historical contingency in the course of a vast sex contest.

Partly because of his experience with barnacles Darwin became increasingly aware of the importance and pervasiveness of sex. The barnacles, although generally hermaphroditic, turned out to cross-fertilize. Turning his attention to plants, Darwin found out that they cross far more often than anybody had realized. Indeed, there were widespread adaptations furthering outcrossing. He undertook extensive empirical investigations, with striking results that revolutionized floral biology. His first book after the Origin of Species (Darwin 1862) dealt with the pollination biology of orchids. It documented in great detail the often complex adaptations that enabled the orchids to have insects transfer pollen from one flower to another. These adaptations were often quite bizarre and the book can at least be read as a sort of satire on traditional natural theology. It also counteracted the morphological tradition that had tended to dismiss function. The book evoked a debate between Darwin and his supporter the Harvard botanist Asa Gray (1810–88). Darwin firmly resisted Gray’s efforts to find God superintending the overall trend of evolution. In the orchid book and other botanical publications, Darwin showed that inbreeding could be highly deleterious and that plants have both anatomical and physiological adaptations that further outcrossing.

In The Descent of Man, and Selection in Relation to Sex (Darwin 1871), Darwin laid the foundation for a great deal of work on the comparative biology of reproduction. His theory of sexual selection followed from his basic idea that evolution results from reproductive competition between individuals. Such features as the antlers of stags and gaudy plumage in male birds he interpreted as adaptations used in monopolizing mates, and not as contributing to survival or otherwise giving an advantage in competing with organisms of other species.

Individualistic competition also has a great deal to do with another important topic in that book: the evolution of morals. Darwin was quite clear on the point that organisms generally act in their own interest. He even suggested that an adaptation in one species that existed solely for the good of another species could not be explained in terms of his theory. He likewise ruled out adaptations that would benefit a species as such. There was as he saw it no good evidence that such adaptations exist. However, he still had to account for the existence of self-sacrifice upon the part of social organisms, including apparently altruistic acts by human beings. His discussions of such topics include basically everything that was considered by late twentieth-century sociobiologists but without the genetics. He seems to have understood kin selection, insofar as he pointed out that in artificial selection traits could be favored by what Darwin called ‘breeding from the same stock.’ A relative of an animal with a trait such as flavorful meat could be selected for breeding instead of that particular animal. He seems not to have considered kin selection very important, however. Neither did he emphasize reciprocity, though he recognized its existence. Rather, he stressed familial selection, which was important in his treatment of the neuter casts of social insects such as ants and termites. He treated families as individuals, with the relevant competition going on between nests of ants or hives of bees. In dealing with human morality Darwin invoked selection at the level of the tribe or nation. His reasons for invoking such group selection, rather than, say, reciprocity, are not explicitly stated in his writings.

To what extent did Darwin go along with the ‘social Darwinism’ of his times? This term is by no means monolithic and merely saying that he endorsed some of it hardly answers the question. Social Darwinism was basically a political doctrine and it generally had a basis that was if not religious, at least teleological. Interfering with the struggle for existence would tend to prevent the desirable end result from happening. It would be like interfering with the growth and matu-ration of an organism. Darwin did not endorse such claims. However, he did maintain that free com-petition favored both biological and economic progress, the two being interrelated insofar as the human intellect is a source of innovation. Lessening the struggle would, he thought, tend to be dysgenic.

There was also a psychological aspect to Darwin’s thinking about morals, one that often focused upon the ‘moral sentiments,’ as Adam Smith had called them. He tried to derive those sentiments from a feeling of sympathy. He considered morality highly serviceable in the struggle for existence, but the selective context was crucial. He suggested that in bees, in which the drones are killed by the workers, it would be appropriate for such fratricide to be a matter of conscience. Some of his readers were scandalized. Selection favoring instinctive behavior was important here, and it contrasts with much of the thinking of his contemporaries. Herbert Spencer had attributed moral sentiments to a more Lamarckian mechanism. There was a kind of racial memory of the sufferings of our ancestors. The British empiricist tradition had sought to explain ethics in terms of personal experience. Spencer made it racial experience. Darwin replaced experience with reproductive success.

The Descent of Man, and Selection in Relation to Sex (Darwin 1871) was followed by its sequel, The Expression of the Emotions in Man and Animals (Darwin 1872), which further elaborated upon Darwin’s evolutionary psychology. Darwin rejected mind–body dualism and thought that the functioning of the nervous system and the rest of the body were very tightly linked. He used facial expression and other body movements as a means of gaining access to the inner workings of the nervous system. It also provided examples of evolutionary continuity. As he saw it, emotional expressions might come to function in communication, but in keeping with his general anti-teleological position they did not originate that way. Rather, they could be traced back to historical antecedents having a different functional significance. Darwin’s interest in psychology can be traced back to his undergraduate years and was a major topic for his empirical research. Two features of this work need to be stressed at the outset. First, although he owed much to the extensive literature on such topics, with which he was quite familiar, much of his contribution came from rejecting the assumptions that had often been made. For example, he rejected the (teleological) notion of Charles Bell that there are anatomical adaptations for expressing emotions. Second, his work in that area rested upon a solid basis in experience, including laboratory experiments as well as natural history observations, with much attention paid to the underlying physiological mechanisms. This at a time when psychology was just beginning to disassociate itself from philosophy and become a laboratory science.

Much of Darwin’s behavioral research was done on plants. One naturally hesitates to call such work ‘ethology’ or ‘physiological psychology,’ but the lack of muscles and nerves in plants does not prevent them from moving and responding to stimuli. Bridging the gap between plants and animals was part of Darwin’s effort to overturn traditional metaphysics. The book on insectivorous plants does more than just allude to that point. In his work on climbing plants Darwin found a morphological continuity between the organs used in climbing and less modified precursors. Tendrils were the modified petioles of leaves, for example. He also found that he could trace the circling movements with which the plants located objects upon which they climbed to a circling movement that had no such function. That led him to investigate the manner in which plants respond to directional stimuli. His research on that topic was first-rate plant physiology and gave rise to a vast literature. His follower George John Romanes (1848–94) extended that sort of reasoning to simple marine animals and thereby founded an important tradition in the study of animal behavior.

Darwin also concerned himself with the ontogeny of behavior, especially that of human beings. He kept detailed notes on the behavioral maturation of his own children. Some of this material was incorporated in The Expression of the Emotions in Man and Animals (Darwin 1872). A paper containing excerpts from the notes (Darwin 1877a) is considered one of the classics of child psychology. Darwin had several reasons for studying behavioral ontogeny. Not the least is the light it may cast on the role of inheritance vs. learning. If there has been no opportunity for learning, and behavior appears spontaneously in the course of maturation, then a strong case can be made for inheritance.

Another reason for studying the ontogeny of behavior was Darwin’s basic position on the relation between development and evolution in general. For him, the modification of ontogeny was a fundamental evolutionary process, and he routinely approached evolutionary problems from the point of view of embryology. Darwin believed that the processes responsible for the development of a a fertilized egg into an adult are something that themselves evolve. This was in itself an important innovation. Perhaps more importantly he showed how natural selection could modify the structure and function of organisms by affecting the various stages of development. By implication, the history of life could be approached by asking how the underlying developmental processes had been modified. His great follower Fritz Muller (1821–97) developed Darwin’s approach by suggesting that the history of certain crustaceans could be interpreted as the result of stages being added at the end of the life cycle. As a consequence, the most modified animals would repeat, during their life cycles, the series of adult ancestors. Another of Darwin’s followers, Ernst Haeckel (1834–1919), took Muller’s special case and treated it as the general rule, and enunciated what he called the fundamental biogenetic law, according to which ontogeny recapitulates phylogeny.

Darwin’s views on the relationship between development and evolution are discussed at length in The Variation of Animals and Plants under Domestication (Darwin 1868). The variations that play an indispensable role in natural selection are here shown to occur in a more or less orderly fashion, evidently as a result of the modification of existing developmental mechanisms. Variation was thus physiologically constrained, and evolution by natural selection was by no means affected only by external conditions as is so often maintained in caricatures of Darwinism. Because this book contains toward its end an effort to explain heredity as well as development, the work is often misinterpreted as mainly about genetics. Darwin attempted to explain the inheritance of acquired characteristics (and much else as well) in terms of particles that control development. These particles could be affected in the somatic tissues, not just the germ. The particles interested some of Darwin’s followers and, with considerable modification in the hands of Hugo De Vries and others, they evolved into the ‘genes’ of later authors.

3. Darwin’s Relevance For Today

Darwin was a remarkably creative thinker, as attested by the surprise with which his discoveries were so often met. One reason for this was the sort of wide-ranging curiosity and erudition that has become increasingly difficult to emulate as knowledge becomes ever more specialized. He was a very good observer, but insisted (at least in private) that this was because his observations were guided by theory. On the other hand, he was also an empiricist, and did not sympathize with the ‘deductive’ approach as he called it of such authors as Herbert Spencer. He confronted his theories with a mass of evidence and modified them in the course of investigation.

Darwin was not constrained by academic fashion or pedantry in seeking material that could be put to scientific use. He unearthed a wealth of information in the writings of plant and animal breeders and appealed to gardeners and others to provide additional materials. He also carried out extensive series of observations and experiments, often opening up new areas for investigation. We should mention his studies of competition between plants as pioneering contributions to experimental ecology. Darwin’s experimental work is very highly regarded by plant physiologists and students of animal behavior. However, his originality so far as methodology is concerned has more to do with comparison than with experiment. In Darwin’s hands, comparative biology became linked to the revolution in systematics, a discipline which was now based upon genealogical relationships and there-fore fundamentally historical. Hypotheses could be developed and tested by such procedures as plotting data onto a phylogenetic tree. Phylogeny was especially important with respect to adaptation, which was now conceived of as the outcome of an historical process.

The shift to historical narrative is an aspect of the rejection of traditional, teleological modes of thinking in which states of adaptation had implied purposefulness, even conscious design. Although structures might give many of the appearances of having been constructed by an intelligent being, it was no longer legitimate to treat ours as the best of all possible worlds. By taking an historical approach and under-standing the actual processes and events that have brought about the present properties of organisms we can avoid the teleology. Instead of asking what an organ is ‘for’ we ask what has brought it into being. It then becomes apparent that the world abounds in maladaptations and historical vestiges.

Contrary to what has sometimes been maintained, natural selection is not capable of explaining just about any conceivable property of organisms. Indeed the mechanism places strong restrictions upon the actual course of evolutionary change. For that very reason, it provides a powerful tool for evaluating hypotheses, especially where adaptation is involved. There is nothing that might reasonably be considered analogous to foresight when evolution occurs by natural selection. Any ‘preparation’ for future conditions of existence has to be the result of coincidence, and populations remain adjusted to present circumstances, and/organisms repeat that which furthered their ancestors’ reproductive success.

Because natural selection depends upon individualistic competition, there are strong constraints upon the kind of adaptation that can occur above the level of the organism. As already mentioned, Darwin clearly recognized that the individuals that reproduce differentially can be families as well as organisms, and on that basis he was able to explain the evolution of neuter castes in social insects. How readily that can happen has been a serious topic of controversy ever since. Darwin did not believe that differential re-production of species could produce adaptations that exist ‘for the good of the species,’ though the possibility has subsequently been taken seriously by some authors. ‘Group selection’ was widely invoked in the early twentieth century. A critical attitude toward it did not become common until the latter half of that century, when in fact there would seem to have been something of an over-reaction. From the point of view of Darwin’s contribution, however, what matters is not the ultimate answers that may emerge, but rather the manner of framing the question. One needs to ask precisely how the world is structured and how the components might interact, formulate explicit hypo-theses and then test them.

Such issues of course are particularly important in the study of social behavior. The modern discussion of what sociobiologists call ‘altruism’ is a continuation of Darwin’s pioneering work in that area. It is important, however, to recognize that ‘altruism’ in this sense is a technical term without ethical connotations and means self-sacrifice pure and simple. But people habitually slip back into older ways of thinking and tend to attach value judgments to statements of fact on the assumption that what is ‘natural’ is therefore good. Even where the teleology is repudiated explicitly it may persist in the form of implicit assumptions. It is clearly an over-reaction, however, to reject the study of adaptation in favor of so-called proximate causes. ‘Teleology’ in the limited sense of homing behavior and the capacity for foresight and understanding on the part of human beings and some other animals is by no means inconsistent with the expectations of selection theory.

Darwin recognized that natural selection is capable of accounting for the phenomena of progress. He reasoned that in a competitive economy innovations that are effective in dealing with the environment would tend to accumulate. On the other hand, he rejected the views of many of his predecessors and contemporaries whose conceptions of progress remained teleological. Notions about evolution, whether social, economic, or biological, based upon analogies with the growth of crystals or the development of embryos implied a kind of inevitability that was out of line with Darwin’s theory. But they appealed to those who were looking for some kind of long-term goal to it all. Darwin witnessed a great deal of technological progress in his day, and like the overwhelming majority of scientists then and now, he obviously believed in scientific progress as well. Although the idea of progress has recently come under attack by philosophers and social scientists, this may just be another episode in the ongoing process of coming to grips with Darwin’s contribution.

Bibliography:

1. Browne J 1995 Charles Darwin: Voyaging: Volume One of a Biography. Alfred A. Knopf, New York
2. Darwin C 1839 Journal of Researches into the Geology and Natural History of the Various Countries Visited by H.M.S. Beagle, under the Command of Captain Fitzroy, R.N. From 1832 to 1836. Henry Colburn, London
3. Darwin C 1842 The Structure and Distribution of Coral Reefs. Smith and Elder, London
4. Darwin C 1844 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. Smith, Elder, London
5. Darwin C 1846 Geological Observations on South America. Smith, Elder, London
6. Darwin C 1851 A Monograph on the Sub-class Cirripedia, with Figures of all the Species. The Lepadidae; or, Pedunculated Cirripedes. Ray Society, London
7. Darwin C 1854 A Monograph on the Sub-class Cirripedia, with Figures of all the Species. The Balanidae (or Sessile Cirripedes); the Verrucidae, Etc., Etc., Etc. Ray Society, London
8. Darwin C 1859 On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. John Murray, London
9. Darwin C 1862 On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing. John Murray, London
10. Darwin C 1865 On the movement and habits of climbing plants. Journal of the Linnean Society of London (Botany) 9: 1–118
11. Darwin C 1868 The Variation of Animals and Plants under Domestication. John Murray, London
12. Darwin C 1871 The Descent of Man, and Selection in Relation to Sex. John Murray, London
13. Darwin C 1872 The Expression of the Emotions in Man and Animals. John Murray, London
14. Darwin C 1875 Insectivorous Plants. John Murray, London
15. Darwin C 1876 The Effects of Cross and Self Fertilisation in the Vegetable Kingdom. John Murray, London
16. Darwin C 1877a A biographical sketch of an infant. Mind 2: 285–294
17. Darwin C 1877b The Different Forms of Flowers on Plants of the Same Species. John Murray, London
18. Darwin C 1880 The Power of Movement in Plants. John Murray, London
19. Darwin C 1881 The Formation of Vegetable Mould, through the Action of Worms, with Observations on their Habits. John Murray, London
20. Goldie P, Ghiselin M T 1996 Darwin Multimedia CD-ROM, 2nd edn. Lightbinders, San Francisco
21. Lyell C 1830–32 Principles of Geology, being an Attempt to Explain the Former Changes of the Earth’s Surface, by Reference to Causes now in Operation. Murray, London
22. Malthus T R 1826 An Essay on the Principle of Population. Murray, London