Was Darwin an ecologist?

I gave two seeds to a confounded old cock, but his gizzard ground them up; at least I cd. not find them during 48o in his excrement. Please Mr. Deputy-Wriggler explain to me why these seeds & pods, hang long & look gorgeous, if Birds only grind up the seeds, for I do not suppose they can be covered with any pulp.— Can they be disseminated like acorns merely by birds accidentally dropping them. The case is a sore puzzle to me.—

Charles Darwin to J. D. Hooker, 10 December [1866]. See the letter

The ‘hard seed for grit’ hypothesis … predicts that seeds defected or regurgitated by birds with non-muscular gizzards (e.g. toucans) would have lower germination rates than those defecated or regurgitated by birds with muscular gizzards (e.g. Galliformes). To test this prediction, all seeds defecated or regurgitated by guans … and toucans … in the aviary experiments were collected and tested for germination in the greenhouse.

Mauro Galetti, 2002: ‘Seed dispersal of mimetic seeds: parasitism, mutualism, aposematism or exaptation?’, pp. 182–3. See the paper

A large, cream-coloured house with trellises stands beyond a mown lawn.One of the most fascinating aspects of Charles Darwin’s correspondence is the extent to which the experiments he performed at his home in Down, in the English county of Kent, seem to prefigure modern scientific work in ecology. Despite the difference in language between Darwin’s letter and the modern scientific paper quoted above (and even between Darwin’s more formal, published, writings and modern scientific papers), the coincidence of images – Darwin in the 1860s following chickens around for two days, and ecology research students in the late twentieth/early twenty-first century doing much the same – is striking. The coincidence is all the more intriguing since they were essentially working on the same puzzle: the existence of bright colours in seeds that have no nutritive value. Other subjects that Darwin worked on at Down also have ecological resonances: the activities of earthworms; the mix of species in a plot of grass; pollination. Was Darwin, then, an early ecologist?

The answer to this question is far from straightforward. As we shall see, though Darwin’s work was pivotal – and in more ways than one – in establishing the modern field of ecology, the assumptions and frameworks that he worked within were very different from the ones we tend to take for granted today. Ecology as a discipline did not then exist: even the word was not coined until 1866. There was no academic department that really covered the same ground. At the same time, sophisticated laboratory-based science was becoming well enough established in universities that Darwin’s ‘held together with a piece of string’ experiments could seem suspect to some observers. For example, the German botanist Julius von Sachs, who headed a state-of-the-art laboratory institute in Würzburg, criticised Darwin’s experiments on movement in root radicles as ‘unskilfully made and improperly explained’ (quoted in Chadarevian 1996, pp. 17–18). As a gentleman amateur, observing his surroundings, Darwin seems to fit easily into an earlier tradition of natural history; yet the kind of experiments that his theory inspired – experiments to do with the relationships between organisms over time – were highly innovative. Darwin’s own experiments challenged the old, purely observational tradition of natural history, and at the same time also challenged the notion that only a laboratory could serve as the place in which valuable experimental work could be performed. He brought his experiments into the natural world and inspired an experimental tradition in the field.

Two ladies in casual summer clothes busy themselves with a number of labelled bags containing soil, amidst several earthen beds.

Magnifying glass icon. Modern ecologists in Brazil plant scat from birds fed bright seeds

Modern ecology

A great deal is wrapped up in our modern idea of ecology. Ecological or environmental philosophy studies the values we give or might give to the natural world and tries to establish or criticise the ethical systems according to which we interact with the natural world. Ecological or environmental history studies the changing ways in which humans have viewed and interacted with the natural world. Ecological science studies the relationships of living things to each other and to their environment using up-to-date scientific methodologies. The ecological movement, which rose to prominence in the 1970s, and which draws on the other three strands just mentioned, is a broadly based political movement which is concerned with the effect of human activity on the environment and which advocates specific policies to mitigate such effects or render them more benign. Key texts of the ecological movement, such as Rachel Carson’s Silent spring, often draw on science, philosophy, and history in order to establish an argument for action. When we think about ecology in the past, therefore, we have to bear in mind that it is an idea – or set of ideas – with many roots, and a correspondingly complex history.

What’s in a name?

The term ‘ecology’ was coined by the German scientist and theorist Ernst Haeckel in 1866. ‘By ecology, we mean the whole science of the relations of the organism to the environment including, in the broad sense, all the “conditions of existence.’ (Ernst Haeckel, Generelle Morphologie 2: 286; translation from Stauffer 1957, p. 140.) The creation of the term ‘ecology’ clearly did not mark an epoch in the history of science; Darwin and some of his correspondents complained mildly about Haeckel’s propensity for making up words, but did not quarrel about the sense behind his definitions. ‘The number of new words … is something dreadful’, Darwin wrote to T. H. Huxley on 22 December 1866. ‘He seems to have a passion for defining, I daresay very well, & for coining new words.’ See the letter

The word first appeared in English in E. Ray Lankester’s translation of Haeckel’s History of creation in 1876; it was slow to catch on, with societies for and university departments of ecology being set up in the English-speaking world only from the late nineteenth century onwards. Darwin himself never used the word, either in his published works or in his letters. However, Darwin’s Origin of species was Haeckel’s primary inspiration for his description of the field of what he called ecology. ‘Ecology is the study of all those complex interrelations referred to by Darwin as the conditions of the struggle for existence’ (Inaugural lecture 1869; translation by W. C. Allee quoted in Stauffer 1957, p. 141).

A monochrome photographic portrait showing a youngish man with a short, curly beard. He is waering a shirt with collar, a jacket, and a strikingly large neckerchief.
Ernst Haeckel

How important is it that Darwin did not use the word ecology, and that it was barely recognised as significant in his time? In one sense it seems not to be very important: we can decide what we consider to be ecology, look into the past for people doing just that, and call it, if not ecology, then perhaps a precursor to ecology. People had been thinking about the relations of organisms to their environment for some time before Haeckel thought of a word for the activity; such thoughts might have been put, in the English-speaking world, under the heading of ‘natural history’, or ‘the economy of nature’.

In another sense though, it is important. When we try to understand what people do, a grasp of what they think they are doing usually plays an important part. Since scientists tend to identify strongly with particular communities and traditions, comparing themselves with and testing themselves against established conventions, institutional and disciplinary boundaries in science are important. These boundaries have changed in the past and will no doubt continue to change in the future. Indeed, when Haeckel coined the term ecology he intended it as part of a redrawing of disciplinary boundaries within the fields of natural history and biology. In his view, the academic discipline of physiology had neglected the relationships between organisms and their environment, and left such study to an ‘uncritical’ natural history (Haeckel 1866, 2: 286–7; see also Stauffer 1957, p. 141). Our modern ecological science is descended from a combination of Haeckel’s ecology and another new science of his, chorology (population distribution).

We can appreciate from our own point of view how important it might be to retain a sense of past categories. Suppose that in a hundred years’ time a new ‘ology’ arises that draws on bits and pieces of various current disciplines. Imagine historians of the future looking at our physics, biology, and chemistry and announcing that though oddly misdirected, and encumbered with puzzling irrelevancies, all of these disciplines contained precursors of their modern ‘ology’. Though these historians might be right as far as identifying components of the future ‘ology’ was concerned, we might feel that they were missing the point about the actual aims and contexts of our old sciences. We perhaps never knew we were heading for that particular ‘ology’ and not knowing that, we could be said not to have been heading that way at all.

So, we should be careful not to make the same mistake with Darwin. When we try to understand the development of the various strands of ecology, Darwin will be found to play an important part; there is, without doubt, a vertical dimension to the story. But there’s also a horizontal dimension, the question of what Darwin himself thought he was doing.

In order to understand ecology historically, it’s necessary to understand something about the ideas that were associated with it in the Victorian period as well as something about the ideas that led up to it. The meanings of words, and of ideas, change over time, and just because we use the same words as the Victorians did, it does not follow that we mean the same things by them. In order to grasp what ecology and related ideas meant to the Victorians, we have to look at how they fitted into a whole network of ideas. Doing this helps us to understand the ideas that are wrapped up in our modern notion of ecology and ecological science: what have we rejected from the Victorian conception of how nature and science work, what have we retained, sometimes without realising it, and what areas are still contested?

Darwin’s intellectual context

A large brick house with bay windows of different periods stands beyond a mown lawn.

Gilbert White’s house in Selborne

Darwin would probably have described himself primarily as a naturalist, which is what he appears as on the list of the supernumeraries on HMS Beagle. In its early use, the term seems to approximate more to our term ‘scientist’: a ‘naturalist’ might be involved in chemical or meteorological investigations. By Darwin’s time the term was associated particularly with people who made collections and catalogues of natural objects: indeed, this is pretty much what Darwin did on the Beagle voyage.

An acknowledged masterpiece of eighteenth-century natural history, and an early influence on Darwin, was Gilbert White’s Natural history of Selborne. In this work, White set out to make personal observations on a relatively small area of English countryside. White laid emphasis on the fact that he had seen things with his own eyes (though he includes anecdotes and the report of others also) and on the small compass of his investigations. He wrote: ‘Monographers, come from whence they may, have, I think, fair pretence to challenge some regard and approbation from the lovers of natural history; for, as no man can alone investigate all the works of nature, these partial works may, each in his department, be more accurate in their discoveries, and freer from errors, than more general writers, and so by degrees may pave the way to universal correct natural history’ (p. 95, 7th edition, 1836). White himself does not spell out why such a universal natural history might be desirable: one might suggest economic motivations as well as the satisfactions of knowlege for its own sake. Indeed, White is a good deal more interested in the possibilities for ‘improvement’ of the yield of the land than modern natural history writers might be. But the editor of the 1836 edition has an equally powerful motivation to suggest, commenting that White’s mind was ‘ever open to the lessons of piety and benevolence, which such a study is so well calculated to afford’ (ibid., p. iv). White was certainly keen to see in nature instances of the wisdom and kindness of its Creator: of the great speckled diver he writes, ‘Every part and proportion of this bird is so incomparably adapted to its mode of life, that in no instance do we see the wisdom of God in the creation to more advantage’ (ibid., p. 296).

White’s viewpoint was not unusual. The existence of God had been for most people a basic assumption that provided an unquestioned foundation for much work in science. In Britain, natural history and natural theology had been intimately linked. In some ways Darwin’s work fitted neatly into the established conventions of natural history; he used his own eyes, he collected, catalogued, and showed the intricate interrelationships between organisms. At the same time, he satisfied the professional conventions that were developing in science; he studied the right books, knew the right people, learnt the right skills, and published in an authoritative manner, choosing at first small areas that he could become thoroughly knowledgable about. He acknowledged the influence of White, and other naturalists, upon him.

Portrait painting of a genial-looking man appearing to be in late middle age. He is wearing a white wig and a red velvet jacket with yellow or gold buttons.

Magnifying glass icon. Carolus Linnaeus (Carl von Linné)

However, Darwin’s theory challenged some of the deepest underlying assumptions of earlier natural historians. Many people believed that the natural world had been created by God in more or less the form it now took, and would not change again until He changed it; until that time, most apparent natural change was thought to be cyclical, usually seasonal; if things changed at all, the same things would come round again and again. Creatures were thought to be adapted to their place in the world in the sense that they had been designed to fill that place and would no doubt continue to do so, unless God himself devised a better plan. The study of nature was, very often, the study of the felicity of various parts of the divine design. When Gilbert White in his Natural history of Selborne described the mole-cricket as having ‘fore-feet curiously adapted to the purpose’ (of burrowing), he used the term ‘adapted’ in the sense of ‘designed’ or ‘suited’. The purpose of the divine design was usually assumed to be the benefit of humankind. To learn more about design in nature, click here

This world-view had begun to be doubted in the eighteenth and nineteenth centuries as a result of geological research that showed evidence of vast upheavals in the earth’s history. Further, fossils were discovered that seemed to show that organisms very different from modern organisms had once walked the earth, and physiological research showed similarities in the structure of what might be thought to be quite unrelated creatures. A picture of change and relatedness was emerging, and many scientists became convinced that species had changed over time.

Accepting species change was not fatal to natural theology: it could be argued that such change was guided by God. But Darwin’s theory, while not commenting on the existence of such guidance, made it strictly speaking unnecessary.

According to Darwin’s theory of natural selection, small variations between organisms would give some an advantage in the struggle for existence. Favoured individuals would produce more offspring, who would inherit their parents’ advantage, and so species would gradually change in the direction of greater and greater adaptation to their environment. As a result of geological and climatological changes, and the pressure on resources from other organisms, the environment would always be changing to some extent, so that the process of adaptation would tend to be unceasing, except when periods of unusual stability occurred, or when species finally died out, or when they occupied a favourable niche so efficiently that there was no pressure for change. According to Darwin’s theory, the natural world showed evidence of linear or open-ended change, not only seasonal or cyclical change: with linear change, past states of the world can never occur again (or at least the odds are vanishingly small).

Photographic monochrome portrait of a lady wearing a dark dress with frilly white bib. She is holding a pen, which rests on a pad of paper on a desk by her side.

Magnifying glass icon. Mary Everest Boole

By establishing a mechanism for species change, Darwin forcefully affirmed the idea of the natural world as a realm of struggle. In doing so he seemed to throw his weight behind ideas from, for example, Thomas Hobbes and Thomas Robert Malthus about the inevitability of violence and conflict in the natural world, including among humans, and challenged the view of nature as orderly, benign, and under continuing divine supervision; an idea put forward by, among others, the influential Swedish naturalist Carolus Linnaeus. To see pain and suffering in the natural world is not, in itself, an argument for atheism, but as Darwin himself acknowledged in a letter to Mary Boole, it was more satisfactory to him to view pain and suffering as an inevitable result of the natural sequence of events than as a result of the direct intervention of God. See the letter

We may contrast Darwin’s discomfort with Linnaeus’s certainty, faced with much the same facts, that the economy of nature was not a kind of capitalist free-for-all (a ‘laissez-faire’ economy in the terms of Darwin’s own society) but what we would now call a command economy. Prey species produced large numbers of young in order that predators might be adequately fed; the brutal-seeming dispensations of nature were in fact wise checks and balances needed to keep the system in equilibrium. But by Darwin’s time, this perspective was becoming difficult to maintain.

Open-ended change

Photograph of a large white flower with extremely long green spurs.

Magnifying glass icon. Angraecum sesquipedale

There are many examples of cyclical change in nature; the turning of the seasons, the birth of new generations, even regular fluctations in populations. However, the theory of evolution through natural selection foregrounds open-ended change: organisms evolve from one form into another; populations increase or decline and may die out altogether. In this context, Darwin’s experiments and observations take on a temporal aspect. Darwin is interested not just in an organism’s adaptation to a static or regularly changing environment but in the ever-changing co-adaptation of different organisms to one another.

A promising example of this is the remarkable co-adaptation of an unusual flower, the comet orchid, Angraecum sesquipedale, with the moth that pollinates it. Darwin discussed this flower in his book On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing (Orchids: 1862), which was the next major work he published after On the origin of species. The impetus for Orchids was Darwin’s perception that many of the strange morphological features of orchid flowers ‘made sense’ if looked at as part of a system to ensure intercrossing. In Orchids, Darwin explained aspects of floral morphology as adaptive behaviour. Further, he argued that the insects that carried the pollen could, to some extent, be predicted by the form of the flower.

Charcoal or pencil sketch of a man appearing to be in early old age. He is wearing small round eyeglasses. His beard has been shaved from the front of his chin, and he has no moustache.

Magnifying glass icon. Joseph Dalton Hooker

Considering Angraecum sesquipedale, Darwin had written to his friend, the botanist Joseph Dalton Hooker, ‘do you know its marvellous nectary 11½ inches long, with nectar only at the extremity. What a proboscis the moth that sucks it, must have! It is a very pretty case.See the letter

Darwin was confident enough to repeat his prediction in print (Orchids, pp. 197–203), and speculated on the co-adaptation of orchid and moth:

As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life, either in the larval or mature state, or as the proboscis alone was lengthened to obtain honey from the Angræcum and other deep tubular flowers, those individual plants of the Angræcum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed, and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angraecum and the proboscis of certain moths; but the Angraecum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as far as possible in order to drain the last drop of nectar.

Orchids, pp. 202–3.

A moth with a very long proboscis approaches a flower with a long nectary. In the background is a tropical vista.

Magnifying glass icon. Hypothetical sphinx moth pollinating Angraecum sesquipedale

The moth was not discovered until 1903, but Darwin’s prediction was correct. In honour of the prediction, the moth, a variety of the species colloquially known as Morgan’s sphinx, was named Xanthopan morgani praedicta: the predicted Morgan’s sphinx. (Rothschild & Jordan, 1903: p. 32.) Learn more about the moth’s prediction and discovery

One of Darwin’s critics, George Campbell, the duke of Argyll, a believer in design, found Darwin’s account of how the structure of this orchid arose profoundly unsatisfactory. Campbell commented on the passage quoted above:

How different from the clearness and the certainty with which Mr Darwin is able to explain to us the use and intention of the various organs! or the primal idea of numerical order and arrangement which governs the whole structure of the flower! It is the same through all Nature. Purpose and intention, or ideas of order based on numerical relations, are what meet us at every turn, and are more or less readily recognised by our own intelligence as corresponding to conceptions familiar to our own minds. We know, too, that these purposes and ideas are not our own, but the ideas and purposes of Another, of One whose manifestations are indeed superhuman and supermaterial, but are not ‘supernatural’, in the sense of being strange to Nature, or in violation of it.

The reign of law, p. 46.

Monochrome photograph of a moth with a very long proboscis, laid out for scientific display.

Magnifying glass icon. Xanthopan morgani praedicta (Kritsky, G. 2001)

It may be difficult for us now to see what is so unclear in Darwin’s account, but Campbell’s distaste is evident. The orderliness of the orchid’s structure was better and more obviously accounted for, in his view, by divine purpose, than as the result of a mindless ‘race’ between a flower and a moth. Although Darwin sees the race as one that the flower ‘wins’, Campbell, in an earlier passage, describes it as a race from which both parties benefit. Nowadays, we are familiar with the idea that what we sometimes call ‘arms races’ between organisms can end up being wasteful and destructive; but Campbell still tends towards an interpretation based on mutual felicity, in the old tradition of natural history. Campbell saw the ‘contrivances’ of the natural world as strictly analogous to human contrivances or machines, and deduced the existence of a governing mind as the only plausible author of them. Beauty, symmetry, and orderliness in nature could then be seen as ends in themselves, as well as, sometimes, being ‘subservient to use’ (The reign of law, p. 197). Since God, according to Campbell, always worked through natural law, ‘scientific’ explanations for natural phenomena would be valid and informative; yet they would be inadequate if due regard were not paid to the purposes at which he aimed through his works. The beauty of humming-birds, for example, could not be accounted for except as a result of the Creator’s fondness for variety and ornamentation (The reign of law, p. 248).

In order to convince Campbell and his other critics of the power of his theory, Darwin had to show not just a single state of adaptation, but evidence of change over time. For Darwin, the study of adaptation was not, as it generally had been before him, a study of how an organism fitted into a niche in a static pattern; it was instead a study of change. Indeed, Darwin’s theory opened the way for the study of dynamic relationships between organisms, and for the study of self-organising (and occasionally self-disorganising) systems. Such studies are pre-eminently ecological. Although many ecological studies do concentrate on single states of adaptation in much the same way as Gilbert White might have, their orientation is different; they are not seeking to uncover an example of the fittingness of the divine plan, so much as taking a snapshot of a moment in a changing history. This orientation was made possible and prefigured by Darwin’s work.

Darwin’s experiments

Darwin’s theory is so important in the history of ecology that it is easy to think of Darwin himself as primarily a grand, and confident, theorist. However, this view does not really do him justice. Through his correspondence, we are able to observe Darwin’s experimental practices in much greater detail than is possible using his published works alone. Through the correspondence, also, we are able to see Darwin’s great theoretical modesty, and his doubts. In fact, Darwin’s experimental programme is as important to the history of ecology as a science as his theory is to ecology as a philosophy. Yet the two cannot be easily disentangled: his experimental programme was distinctive because of the way his theory changed earlier assumptions about how the world worked.

Darwin’s caution is evident in his correspondence with Haeckel, himself a passionate theorist who revelled in the political and theological upheaval that he was confident Darwin’s work would cause. Haeckel acknowledged himself to have been profoundly influenced by Darwin. ‘Of all the books I have ever read, not a single one has come even close to making such an overpowering and lasting impression on me, as your theory of the evolution of species’, Haeckel wrote to Darwin on 9 July 1864. ‘In your book I found all at once the harmonious solution of all the fundamental problems that I had continually tried to solve ever since I had come to know nature as she really is.See the letter

It seems from Haeckel’s letter that what most struck him about Darwin’s theory was that it enabled him to explain, as Haeckel thought, ‘the whole of nature all at once’. Haeckel disdained the ‘exclusive study of details and the analysis of particulars’; he was himself particularly interested in the production of genealogies of living organisms, and ultimately in writing a ‘general history of creation’.

Darwin’s response to Haeckel’s request for an account of his great discovery is by contrast extremely modest. In a letter written in 1864 and enigmatically dated ‘Aug. Oct 8th.’, Darwin wrote: ‘it seemed to me probable that allied species were descended from a common parent. But for some years I could not conceive how each form became so excellently adapted to its habits of life. I then began systematically to study domestic productions, & after a time saw clearly that man’s selective power was the most important agent. I was prepared from having studied the habits of animals to appreciate the struggle for existence, & my work in Geology gave me some idea of the lapse of past time. Therefore when I happened to read “Malthus on population” the idea of Natural selection flashed on me.See the letter

In fact it must have been obvious to everyone except, perhaps, Haeckel himself, that Darwin’s aims and methods were very different from Haeckel’s own. Darwin tends to begin with rather modest claims (that species that seem related in fact are related), and relies on a great deal of detailed work before making any larger – but often still carefully limited – claims. Darwin took it for granted that unless he could establish his authority beyond reasonable doubt in one area, however small, he would not be taken seriously when he made larger claims. Consequently, he did a great deal of detailed experimental work on, for example, barnacles, and in botany. In the correspondence between Haeckel and Darwin, the importance of Darwin’s own scrupulous and cautious progress from observation, to hypothesis, to experiment, to theory (a path that might be retrodden and revised many times) is thrown into relief.

List of persons, references and sources

  • Boole, Mary Everest. Mathematician, librarian and teacher.
  • Campbell, George Douglas (duke of Argyll). Scottish statesman and author.
  • Campbell, George Douglas. 1867. The reign of law. London: Alexander Strahan.
  • Carson, Rachel. 1963. Silent spring. London: Hamilton.
  • Chadarevian, Soraya de. 1996. Laboratory science versus country-house experiments. The controversy between Julius Sachs and Charles Darwin. British Journal for the History of Science 29: 17–41.
  • Darwin, Charles. 1859. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life . London: John Murray.
  • Darwin, Charles. 1862. On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing. London: John Murray.
  • Darwin, Charles. 1865. On the movements and habits of climbing plants . By Charles Darwin. London: Longman, Green, Longman, Roberts & Green; Williams & Norgate.
  • Darwin, Charles. 1875. Insectivorous plants . London: John Murray.
  • Galetti, M. 2002. Seed dispersal of mimetic seeds: parasitism, mutualism, aposematism or exaptation? in Seed dispersal and frugivory: ecology, evolution and conservation, edited by D. Levey, et al. New York: CABI Publishing.
  • Haeckel, Ernst. German zoologist.
  • Haeckel, Ernst. 1866. Generelle Morphologie der Organismen. Allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-Theorie. 2 vols. Berlin: Georg Reimer.
  • Haeckel, Ernst. 1876. The history of creation: or the development of the earth and its inhabitants by the action of natural causes. A popular exposition of the doctrine of evolution in general, and that of Darwin, Goethe, and Lamarck in particular. Translation of Haeckel’s Natürliche Schöpfungsgeschichte. Translation revised by E. Ray Lankester. 2 vols. London: Henry S. King & Co.
  • Hobbes, Thomas. Philosopher.
  • Hooker, Joseph Dalton. Botanist, and close friend of Charles Darwin.
  • Huxley, Thomas Henry. Zoologist.
  • Kritsky, Gene. 2001. Darwin’s Madagascan hawk moth prediction. American Entomologist 37: pp. 206-210
  • Lankester, Edwin Ray. Zoologist, and translator of Haeckel’s Natürliche Schöpfungsgeschichte.
  • Lindley, John. 1853. The vegetable kingdom; or, the structure, classification, and uses of plants, illustrated upon the natural system. 3rd edition with corrections and additional genera. London: Bradbury & Evans.
  • Linnaeus, Carolus. Swedish botanist and zoologist; reformer of scientific nomenclature.
  • McIntosh, Robert P. 1985. The background of ecology: concept and theory. Cambridge: Cambridge University Press.
  • Malthus, Thomas Robert. Clergyman and political economist.
  • Malthus, Thomas Robert. 1826. An essay on the principle of population; or, a view of its past and present effects on human happiness; with an inquiry into our prospects respecting the future removal or mitigation of the evils which it occasions. 6th edition. 2 vols. London: John Murray.
  • Nilsson, L. A. 1998. Deep flowers for long tongues. Trends in Ecology & Evolution 13: 259–60.
  • Richards, Robert J. 2008. The tragic sense of life: Ernst Haeckel and the struggle over evolutionary thought. Chicago: University of Chicago Press.
  • Rothschild, L. W., and Jordan, K. 1903. A revision of the lepidopterous family Sphingidae. Novitates Zoologicae 9 (Suppl.): 1–972. See an extract of the paper
  • Sachs, Julius von. Professor of botany at Würzburg.
  • Sachs, Julius von. 1887. Lectures on the physiology of plants. Translated by H. M. Ward. Oxford: Clarendon Press.
  • Stauffer, Robert C. 1957. Haeckel, Darwin, and ecology. Quarterly Review of Biology 32: 138–44.
  • White, Gilbert. Naturalist and clergyman.
  • White, Gilbert. 1789. The natural history and antiquities of Selborne, in the county of Southampton. 2 vols. London.
  • Worster, Donald. 1994. Nature’s economy: a history of ecological ideas. Cambridge: Cambridge University Press.