rasmus Darwin started to think about evolutionary ideas when his curiosity was aroused by the discovery of mammoth bones at Harecastle near Kidsgrove, Staffordshire, in the course of digging a tunnel for the Grand Trunk Canal in 1767. These bones had been sent to him by Josiah Wedgewood, who realized that they did not relate to any known species, and Wedgewood wanted his opinion on from which animal they could have come. Darwin thought that they must belong to an extinct species, and in 1769 he came to the conclusion that organisms must have evolved through time.
He also concluded that all organisms have a common ancestor — what he termed a "single filament" — formed in water by natural processes. The Frenchman Maupertuis, whose ideas were not published until 1760, had already theorised that life had originated spontaneously in warm shallow water in 1720. Darwin was probably aware of Maupertuis, but he was to develop his ideas about evolution more than anyone else in his lifetime. Just as Maupertuis had been cautious about broadcasting his ideas for fear of the negative reaction that would follow, so Darwin was also reluctant to let others know too clearly of his new insights. He did however use his family coat of arms (which consisted of three scallop shells), by adding the motto: E conchis omnia — everything from shells — which was painted on the doors of his coach and printed on his bookplate. His neighbour in Lichfield cathedral close, one canon Seward, understood the meaning of the design and urged Darwin to remove this offensive emblem from public view, which he did because he realised that if he were to make his evolutionary ideas too well known he would be in real danger of losing many of his patients and thus his source of income. He first described his evolutionary ideas in his poem Economy of Vegetation in which he described life originating through natural processes in warm water and then changing by natural selection. He delayed writing his medical treatise, in which he described his evolutionary ideas in prose, until 1793. His grandson Charles was also to delay publishing his book on natural selection for 20 years in part for fear of a negative reaction.
The first volume of Zoonomia, whose two volumes appeared in 1793 and 1796, includes an extended essay in which Darwin described his ideas about the origin and evolution of life in which natural selection played a dominant role. Here appeared for the first time in English prose the idea of the spontaneous generation of life by physico-chemical means in water when he argued that “it appears that all animals have a similar origin, viz. from a single living filament;" and "All animals therefore, I contend, have a similar cause of their organisation, originating from a single living filament," (p. 498 and p. 499)
Darwin not only believed that that animals had a common ancestor, he also thought that all life had a common ancestry:
Shall we then say that the vegetable living filament was originally different from that of each tribe of animals above described? And that the productive living filament of each of these tribes was different originally from each other……..shall we conjecture, that one and the same kind of living filaments is and has been the cause of all animal life?" 
Then in a passage which has often been overlooked, Darwin argued that the initial living filament for each specific creature is to be found in all parts of it:
All animals therefore, I contend, have a similar cause of their organisation, originating from a single living filament, endued with different kinds of animal appetencies; which exist in every gland, and in every moving organ of the body, and are as essential to living organisations as chemical affinities are to certain combinations of inanimate matter. 
Darwin understood the need for what in modern language would be called DNA: eighteenth-century knowledge did not extend to the molecular level in cells, but Darwin appreciated that something similar must be present in all parts of organisms that differentiated animated matter from inanimate. In making this clear distinction between organisms and non-organic matter, he rejected an idea common at the time and since that organisms are fundamentally machines, or that they can be regarded as machine-like entities.
He identified three objects of desire that would cause animals to change their form: lust, hunger, and survival. Lust led him to define sexual selection. Thus, males of some species desire
the exclusive possession of females; and these have acquired weapons to combat each other for this purpose. . . . So the horns of the stag are sharp to offend his adversary, but are branched for the purpose of parrying or receiving the thrusts of horns similar to his own, and have therefore been formed for the purpose of combatting other stags for the exclusive possession of the females. The final cause of this contest among the males seems to be, that the strongest and most active animal should propagate the species, which should thence become improved."
In this sentence "final cause" should be understood in the Aristotelean sense of the purpose for which something happens, in this case, sexual selection..
The necessity "of procuring food" had "diversified the forms of all species of animals". He described how pigs have hard noses to grub in the ground, elephants have trunks to reach branches and leaves, cattle have rough tongues to pull grass, and birds have a variety of beaks: "Some birds have acquired harder beaks to crack nuts, as the parrot. Others have acquired beaks to break the harder seeds, as sparrows. Others for the softer seeds of flowers, or the buds of trees, as the finches. . . All which seem to have been gradually produced during the many generations by the perpetual endeavour of the creatures to supply the want of food" (504).
Darwin also described how animals had adapted limbs to defend themselves, either by running away from enemies or by concealing themselves:
Some have acquired wings instead of legs, as the smaller birds, for the purpose of escape…Others great swiftness of foot, as the hare. Others have acquired hard or armed shells, as the tortoise and the amphibious echinus marines….The colours of many animals seem adapted to their purposes of concealing themselves either to avoid danger, or to spring on their prey."(p504 and p509). In the case of birds he noted how they laid eggs of different colours according to whether they were in hedgerows, in which case they were green, if seen from below as with crows and magpies they are blue or white, or ground nesting birds which lay grey or brown eggs. (p510) In the above passages Darwin was describing the results of natural selection.
He also understood mimicry as a form of protection: "frog-fish, Lophus Histrio, which inhabits large floating islands of seaweed above the Cape of Good Hope, and has fulcra resembling leaves, that the fishes of prey may mistake it for sea-weed, which it inhabits."
Darwin, who recognised the common anatomical structures of all warm-blooded animals, and concluded:
From thus meditating on the great similarity of the structure of warm blooded animals, and at the same time of the great changes they undergo both before and after their nativity; and by considering in how minute a portion of time many of the changes of animal above described have been produced; would it be too bold to imagine, that in the great length of time, since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament….with the powers of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down these improvements by generation to posterity, world without end? 
Later Cuvier, Jean-Baptiste Lamarck, Geofffroy and Richard Owen would use similar anatomical structures to argue for and against transformism (as evolution was called in the early decades of the nineteenth century), but eventually Darwin's interpretation came to be accepted as an important line of evidence for the fact of evolution.
He did not know what the "powers" were that caused organisms to acquire new parts over time, but he clearly recognised that such changes have taken place, and in doing he disagreed with the majority of his contemporaries. His belief in "irritations, sensations, volitions, and associations" as causes of change within organisms was vague and can be compared with Lamark's later suggestion of an organism's "incidental need" or "modifying cause," which also failed to explain how any change originated or grew. He drew his readers’ attention to frogs and butterflies, which undergo metamorphosis as part of their life cycles. He also pointed out how some animals have been changed by "accidental or artificial cultivation, as in horses, which have exercised for the different purposes of strength or swiftness, in carrying burthens or in running races" (500). He supplied a list of animals — pigeons, dogs, cattle, sheep and camels — which had been modified by cross-breeding to better suit the needs of the breeders. He also noted that monstrosities (mutations in today's language), could be inherited: "Many of these enormities of shape are propagated, and continued as a variety at least, if not as a new species of animal. I have seen a breed of cats with an additional claw on every foot; of poultry also with an additional claw, and with wings to their feet" (501). All of these ideas and examples re-appeared in later volumes published after 1859 by his grandson Charles.
Like his contemporaries who believed in natural theology, Darwin was clearly an adaptationist, but unlike most modern biologists who support adaptationism, he never used terms such as design, mechanism, or any other terms borrowed from physics or engineering when referring to organisms or living entities. The key difference between Darwin and the theologians was that he was an atheist by the time he came to write Zoonomia: he was a materialist who had adopted some of his ideas from Maupertuis. Darwin presented natural theology without God, inverting the adaptationist ideas of natural theology to which he added to the idea of self-organisation and common ancestry, thereby anticipating ideas which were not to be widely discussed again until nearly 200 years later. He recognised that all changes within the physical universe were entirely caused by natural processes. He understood that organisms interact with each other and in doing so cause mutual changes, and although he did not understand the world as a global system, he realised that local communities had to be recognised as greater than the sum of their parts. Zoonomia, which was a great success even though it was a medical treatise, appeared in translations in German, French and Italian and printed in the USA, and served to spread his evolutionary ideas into educated circles across Europe and North America at the end of the eighteenth- and well into the nineteenth-centuries.
It was also put on the Index of books prohibited to Roman Catholics by the Pope. It may have influenced the great French evolutionist Jean-Baptiste de Lamarck, who began teaching courses on evolution in 1802: King-Hele, his biographer, thinks that it influenced Lamark's Philosophie Zoologique of 1809 in which he presented the first laws of evolution: certainly Lamark quoted examples that also appear in Zoonomia, such as the enlarged muscles of blacksmiths. Evolution was a popular idea in the nineteenth century among political and social radicals in many countries other than Britain, and Darwin's and Lamark's names became synonymous with the idea, but it remained an anathema to social and political conservatives for several decades, and both he and Lamark were considered to be heretics because conservatives believed that evolution called into question both the Creation and the existence of God. Believing that society and the world had been created by God, they found any suggestion that they could or should change blasphemous. Given that the French revolutionary government made a clear separation between Church and State, and advocated a society based on meritocracy rather than heredity, conservatives in Britain and Europe thought that ideas about evolution inevitably led to violent revolution.
Darwin, who was interested in agriculture and increasing food supplies by cross breeding plants and animals, borrowed the term natural selection from breeders. In 1800 he published the last of his prose works Phytologia; or the Philosophy of Agriculture and Gardening. (120-31). This book, which he organized in three parts, included more of Darwin's important discoveries and descriptions. In the first part he further discussed sexual selection with respect to plants and described how different types of seeds are dispersed (113). He showed how plants breathe through holes in their leaves by covering the undersides of some in wax which then died, although the pores under leaves were not discovered until the nineteenth century . He recognised that plants make sugar, which is an important food source for many forms of life, both plant and animal. In part II he wrote: "This carbonic gas (carbon dioxide)….is the principal food of plants…..Next to carbonic acid …. water seems to afford the principal food of vegetables…. when vegetable leaves are exposed to the sun's light, they seem to give up oxygen gas" (193 and 194). Here Darwin described the process of photosynthesis for the first time, for although others had recognised different parts of the process, he was the first to describe it as a complete system. He explained that in order to grow plants need nitrogen, phosphorus, carbon, and calcium, advocating that these elements should be regularly added to cultivated soils in order to maintain their fertility and increase and to improve the quality of crop and animal yields. He proposed that sewage farms should be built beside towns and cities to process human waste and then use it as manure on fields: an additional benefit of this would be to clean the water supply and remove a source of many contagious diseases. He suggested that tape-worms be introduced to help control rat populations.
In Part III of Phytologia, which dealt with fruits and trees, root crops and the uses of bark, leaves and the wood of trees, points to the savagery of nature: "Such is the condition of organic nature! whose first law might be expressed in the words, "Eat or be Eaten!" and which would seem one great slaughter-house, one universal scene of rapacity and injustice!" (556) Later Herbert Spencer was to express similar views, and Tennyson described "Nature, red in tooth and claw." His grandson Charles would also be reminded of these words and similar ideas from A. von Humboldt when he witnessed the extravagant life in the rain forests of South America. Unlike some of these later commentators, Darwin also believed that animal activities were in part directed towards pleasure, and in an interpretation of life that was different to his more conservative contemporaries Paley and Malthus, he did not believe that the amount of pleasure in the world was balanced by the amount of pain, because he his ideas were not constrained by the theological framework of the clergymen's thinking.
Darwin, Erasmus. The Botanic Garden Part 2 The Loves of the Plants, J. Jackson, Lichfield for J. Johnson, London 1789. Project Gutenberg 4 November 2016.
Darwin E Zoonomia ; or The Laws of Organic Life Part I J. Johnson 1794. span class="website">Archive.org. 4 November 2016.
Darwin, Erasmus. Zoonomia; or The Laws of Organic Life Parts II and III 1796 J. Johnson 1798. Archive.org. 4 November 2016.
Darwin, Erasmus. Phytologia: or the Philosophy of Agriculture and Gardening, J. Johnson, London 1800. Web version. Summary accessed 4 November 2016.
Darwin, Erasmus. The Temple of Nature, or The Origins of Society, J. Johnson London 1803. Web version. 4 November 2016.
Fara, P, Erasmus Darwin: Sex, Science and Serendipity, Oxford University Press, 2012.
King-Hele, D. Doctor of Revolution, The Life and Genuis of Erasmus Darwin, Faber and Faber, London, 1977.
King-Hele, D. Erasmus Darwin: A Life of Unparalleled Achievement, Giles de la Mare Publishers, London 1999.
King-Hele, D. Erasmus Darwin and Evolution, Stuart Harris, Sheffield, 2014.
Lamarck, Jean-Baptiste Philosophie Zoologique, Paris, 1809. 4 November 2016.
Last modified 10 November 2016