CHAPTER 3

Trees of Life in a Newly Ancient World

Anonymous, Brockhaus and Efron Encyclopedic Dictionary, 1890–1907; Butterflies, beetles and dragonflies.

After Evolution

(1820–1900)

The turning point in the story of zoological classification came with the theory of natural selection – formulated in the middle of the nineteenth century, simultaneously by two British biologists, Charles Darwin and Alfred Russel Wallace.

Natural selection is a satisfyingly but deceptively simple theory: populations of animals exhibit variation in their characteristics; individuals inherit traits from their parents; and if traits confer an advantage on offspring’s ability to thrive and reproduce, then those traits will be preferentially inherited by subsequent generations. Thus, over many years, species gradually develop new traits and change, and sometimes split into daughter-species (‘speciate’), as well. Remarkably, almost all the diversity and complexity of life is now thought to be built upon this process.

The decades before and after the expounding of this theory produced some of the most dramatic visual depictions of animal organization ever created. The images were compelling before natural selection as biologists fought to make sense of an as-yet incomplete model of how the animal world works, and the images were also compelling afterwards as scientists realized how powerful the new biology could be.

Ernst Haeckel (1834–1919), Natürliche Schöpfungsgeschichte, 1868; Single or monophyletic pedigree of the stem of the backboned animals based on palaeontology.

In the decades after Lamarck’s first evolutionary ‘trees’ (see here), other scientists struggled in their attempts to visually represent their proposed classifications of animals. Unable to cast off the young Earth or the unchanging creatures of the Bible, many went to great lengths to create elaborate, pleasingly geometric diagrams that nonetheless merely claimed to illustrate ‘similarities’, ‘affinities’ or the even more cryptic ‘transits’ between animal species. Some looked like exuberant Venn diagrams, some like mysteriously subdivided eggs and others like obscure Kabbalistic Sephiroth. Although inventive and visually appealing, it is hard to see why each of their creators felt so certain that their own personal geometry of animal relationships reflected nature’s true scheme any better than anyone else’s. To modern eyes it can seem that these biologists were doing all they could to avoid drawing what is to us (but not to them, of course) the obvious form – a tree.

During the first half of the nineteenth century, arguments continued to mount in favour of the Earth being much older than stipulated by the Bible narrative. This idea was to be very important for biologists because evolution is probably usually a slow process, so the theory of evolution ‘needs’ an old Earth to be credible. Although not himself the originator of the old-Earth hypothesis, the Scottish geologist Charles Lyell is often viewed as the central character in this geologico-biological concordance. Lyell was a proponent of ‘uniformitarianism’ – the hypothesis that geological conditions, processes, and the forces that cause them, have been largely similar for extremely long periods of time. In his 1830-1833 Principles of Geology, Lyell described his own fieldwork showing that vertically stacked miles of sedimentary strata, and the accumulated conical layers of vast volcanoes, could only have formed over achingly long periods of time, and in accordance with unchanging laws of nature. A copy of the Principles was given to Darwin as he embarked on the Beagle, and Lyell’s model of steady, cumulative geological change was to influence deeply the younger man’s ideas about animals.

William Smith (1769–1839), Geological section from London to Snowdon (map), 1817.

Edward Hitchcock (1793–1864), Elementary Geology, 1840 (see here); Distribution of several orders of Zoophyta.

We now think the Earth is approximately 4.5 billion years old, and has supported life for at least eighty per cent of that time. However, the question of whether the ancient Earth was relatively unchanging or was instead racked by catastrophes has troubled geologists ever since the eighteenth century. Clear evidence of dramatic climate fluctuations was demonstrated by the discovery of past ice ages by the Swiss geologist Louis Agassiz in his 1840 Études sur les Glaciers. Glaciers cut distinctively U-shaped valleys and also transport and deposit ‘erratics’ – large lumps of rock – in the ‘wrong place’ (for example, huge granite boulders perched on top of younger limestone). Agassiz argued that glaciers had once reached as far south as the Caspian and Mediterranean seas, and in 1846 moved to the United States to bolster his theories. Once there, however, his career took an unexpected turn when he became embroiled in arguments about the nature of species, inter-species hybridization and the origin of the human races. He supported the theory of polygenism, the idea that the different human races actually represent different species – and thus is mainly remembered for supporting a theory widely used as a justification for slavery (see Josiah Nott, and his work with George Gliddon, shown here).

John James Audubon (1785–1851), The Birds of America, 1827–1838; Great blue heron.

All the while, the tradition of visually cataloguing animals continued to flourish into the nineteenth century. New territories and new research methods revealed ever more animal diversity to the artistically minded naturalist, and two particular works stand out. The first is John James Audubon’s colossal The Birds of America: published between 1827 and 1838, it contains 435 watercolours exhaustively and lovingly cataloguing the avian inhabitants of the new country. Contributing to these tomes’ weightiness is the fact that Audubon decided to depict his subjects life-size, meaning that each page must be large enough to comfortably accommodate an eagle. The second major work is John Gould and Henry Constantine Richter’s 1845–1863 The Mammals of Australia (see here and here), which exposed European and American readers to the diversity of antipodean marsupial and egg-laying beasts. While not as huge as Audubon’s work, Mammals shows an immensely endearing artistic empathy with its inhabitants. Rarely have such strange creatures looked so proud, furtive or playful. Some of them even appear to have a forlorn resignation in their eye, as if they know that, within a century and a half, some of them will no longer exist.

Throughout the early-to-mid-nineteenth centuries, scientific data accumulated at an unparalleled rate. In 1823, human and mammoth bones were discovered in a Welsh cave, suggesting the two species had coexisted. In 1844, the journalist Robert Chambers anonymously published his controversial Vestiges of a Natural Creation, in which he discussed the formation of the Solar System, the origins of terrestrial life, including humans, and how animal species may ‘branch’ off a stem of ever-increasing developmental complexity. In 1862, Lord Kelvin used the rate of thermal cooling of the Earth to calculate its age to be 20–100 million years, although even these enormous numbers did not satisfy some uniformitarians who claimed the Earth to be infinitely old. Finally, throughout this entire period embryologists and palaeontologists, particularly in France and the German-speaking countries, developed wide-ranging schemes by which the fossil history of animal species, the narrative of animals’ embryonic development, and the new science of evolution might be unified into an all-encompassing scheme of animal life.

Louis Agassiz (1807–1873) and Augustus Gould (1805–1866), Principles of Zoölogy, 1851; Skeleton of a mammoth.

It was in the context of this intellectual ferment that two British naturalists changed biology forever. Charles Darwin was born in 1809, and although it was initially intended that he train as a doctor, his interests soon veered across into natural history. He made his scientific name as the zoologist on the 1831–1836 voyage of HMS Beagle during which he reported on marine fossils left high and dry in mountain rock strata, the bones of extinct fauna found alongside modern-looking mollusc shells, the apparently stable nature of the Earth, a single origin of the human species, and most famously the subtle variations between the finches of the Galapagos Islands. He probably started to formulate his theory of natural selection by 1838, and described it in an unpublished paper in 1844.

Born in 1823, Alfred Russel Wallace (see here) had travelled extensively in Brazil (1848–1852) and what is now Malaysia and Indonesia (1854–1862), documenting and collecting thousands of specimens. It was during the course of these explorations that he began to develop his own theories about natural selection. In 1855 he wrote in tree-like terms of ‘branching lines of affinity, as intricate as the twigs of a gnarled oak’, and began to describe genealogical trees of animal species, and the principles behind drawing them. However, Wallace was as much a geographer as a zoologist, and he discussed animals’ environments, ecology and distribution as enthusiastically as their genealogical interrelationships. Indeed, he was the key instigator of the modern field of biogeography.

Alfred Russel Wallace (1823–1913), The Geographical Distribution of Animals, 1876; The neotropical region.

Darwin and Wallace published together on their new theory in 1858, and encouraged by what he had read in earlier works by Wallace, Darwin published On the Origin of Species on 24 November 1859. Throughout the gestation of the theory of natural selection the two men had encouraged each other along the way, and Darwin certainly emphasized how similar their theories had been. They were to remain lifelong correspondents and mutual supporters, as their big idea began to reshape the scientific world.

Darwin’s notebooks and the Origin contain surprisingly modern-looking schematic trees of animal relationships, although admittedly not many of them. The Origin, for example, contains only one diagram of any kind (see here) – a tree of sorts. He seemed to think the tree motif was already sufficiently established for it to become the preferred form of representation of animals’ relatedness, although he himself suggested that coral might be a better analogy, because coral polyps (like living species) grow atop a substrate of dead ancestors, not a living stem. In fact, perhaps the first published modern evolutionary tree had appeared the year before the Origin, hidden away in a dark corner of Heinrich Georg Bronn’s Entwickelungs-Gesetze der Organischen Welt (see here). In some ways this Teutonic prefiguring of the evolutionary tree was appropriate because, as will become clear, it was another German, Ernst Haeckel (see here), who was to become both Darwin’s most obsessive supporter, and the most avid creator of evolutionary trees in history.

Albert Günther (1830–1914), Report on the Deep-Sea Fishes Collected by H.M.S. Challenger during the Years 1873–1876, 1887; Neobythites grandis (1,875 fathoms) and N. ocellatus (350 fathoms).

John Fleming (1785–1857), The Philosophy of Zoology, 1822; Bats and sea mammals.

John Fleming was a church minister as well as a zoologist, and was never able to reconcile his scientific discoveries with his religious beliefs. Certainly the fact that bats and cetaceans are related mammals strains the credulity of even the most ardent supporter of evolution.

Friedrich Justin Bertuch (1747–1822), Bilderbuch für Kinder, 1805; Squid and octopuses.

The title of Bertuch’s weighty encyclopedia-by-instalments, ‘Picture book for children containing a pleasant collection of animals, plants, flowers, fruits, minerals... appropriate to the intelligence of a child’ belies the quality and beauty of its images.

Carl Eduard von Eichwald (1795–1876), Zoologia Specialis, 1829; Tree of animal life.

Born in present-day Latvia, Eichwald is often credited with creating the first true evolutionary tree, drawing on the written descriptions of Peter Simon Pallas (see here). Indeed, he did write of animals as changing over time, and having their origin in simple marine forms, although his exact meanings are sometimes obscure. The reticence of early eighteenth-century biologists to fully accept that animal groups divide over time is certainly evident in Eichwald’s tree – mainly trunk, with small stubby branches. Thus are the affinities of different species emphasized above their divergence in this weatherbeaten bole.

Henry Thomas De la Beche (1796–1855), Duria Antiquior (lithograph), 1830.

The first, and still one of the most charming, artistic representations of a prehistoric scene, ‘A More Ancient Dorset’ draws on the fossil discoveries and reconstructions of Mary Anning in southwest England. The image is remarkable, not only because it accurately captures the scientific knowledge of the time, but also for its depiction of ancient creatures actually interacting with each other. The largest inhabitant of this part of ancient Dorset is a ravenous ichthyosaur, with a hint of glee on his face as he summarily dispatches a plesiosaur.

Jean-Baptiste Lamarck (1744–1829) and Anna Atkins (1799–1871), Genera of Shells (English translation), 1833.

Although Jean-Baptiste Lamark (see here), the author of this work, is most famous for his contribution to evolutionary theory, his zoological career started and ended with studies of invertebrate diversity. The English translation of this late work was beautifully illustrated by the British botanist, illustrator and later photographer, Anna Atkins, while still in her early twenties (Atkins may be the first woman ever to have taken a photograph.)

Karl Ernst von Baer (1792–1876), Über Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion, 1838; Symmetries in the growth of vertebrates.

The eighteenth century saw great advances in descriptive embryology – not only could species be studied changing over evolutionary time, but individual animals could also be studied developing in utero or in ovo. The Estonian embryologist Karl Ernst von Baer was seminal in realizing that these processes of species evolution and embryonic development must be linked, an interdisciplinary field of study still extremely active today.

William Swainson (1789–1855), A Treatise on the Geography and Classification of Animals, 1835.

Swainson was a proponent of the strange ‘Quinarian System’ of animal classification, by which all animal groups were claimed to be divisible into five subgroups, no more, no fewer. These were represented graphically by interacting circles, whose arrangement corresponded to their members’ affinities and supposed advancement. An entirely unsubstantiated form of zoological numerology, it is not clear why the Quinarian System became even briefly popular.

Edward Hitchcock

The Crown of Creation

TIMELINE

1793: Edward Hitchcock born

1823: Geology of the Connecticut Valley published

1825: Leaves the ministry

1840: Elementary Geology (first edition) published

1864: Edward Hitchcock dies

Edward Hitchcock was the personification of the struggle between science and Christianity in the nineteenth century. After a stint as a devout Congregationalist pastor, he embarked on a successful career in which he was to face, head-on, the alarming inconsistencies between science and theology. In 1825, aged thirty-two, he became professor of Chemistry and Natural History at Amhurst College in Massachusetts, and soon instigated a variety of geological and palaeontological projects, as well as state-wide surveys in the earth sciences.

It rapidly became clear to Hitchcock that the chronology of the Bible fundamentally disagreed with the rocky evidence before his own eyes. In an attempt to reconcile the apparent antiquity of the Earth with the several-thousand-year-old world described in Genesis, he developed his own rather contrived reinterpretation of Bible chronology. He accepted that the six-day creation was a fallacy, and that Noah’s flood was not the mechanism behind rock stratification and fossil deposition (before this time, fossils were often claimed to be antediluvian monsters eradicated by the Creator’s deliberate inundation). Also, using a minor linguistic technicality, Hitchcock claimed that the biblical ‘days’ of creation signified six protracted periods of what we would now call ‘geological time’.

Yet Hitchcock’s god remained the creator and driver of organismal change, and although he depicted animal change and extinction over time in his strikingly tree-like ‘Paleontological Chart’ of 1840 (see opposite), he denied that such change could have solely mundane causes. Also, he was certain man does not share ancestry with the beasts.

Edward Hitchcock (1793–1864), Elementary Geology, 1840; Trilobites.

In the later years of Hitchcock’s life, Darwin and Wallace’s ideas were to make his theological-scientific balancing act collapse. The world is indeed old, but it is the uncaring hand of natural selection which moulds change in those who inhabit it, not the hand of God. After the publication of On The Origin of Species, none of his books ever again contained his elegant and influential ‘Paleontological Chart’.

Edward Hitchcock (1793–1864), Elementary Geology, 1840; Paleontological Chart.

Published in most editions of Hitchcock’s much-reprinted Elementary Geology, his ‘Paleontological Chart’ appears as close as one can come to an evolutionary tree without accepting non-deistic origins and processes for life. Indeed, he did refer to the chart as ‘a tree’. It depicts the radiation of the families of plants (on the left) and animals (on the right) upwards through the span of geological time. Some of the eras are familiar to us today – the Carboniferous and Cretaceous, for example – while others, such as the ‘Saliferous’ and ‘Oolitic’, sound archaic and mysterious. After the initial God-made radiation at the root of each evolutionary ‘bush’, subsequent branching is allowed only when the evolutionary relationships between groups could not reasonably be denied. Unsurprisingly, the (literal) crown of animal creation is assumed to be the Mammalia, but in a strangely arbitrary attempt to give the chart symmetry, it is the palms that are deemed to be monarchs of the plant world.

John James Audubon (1785–1851), The Birds of America, 1827–1838; Flamingo (above); Columbia jay (below).

Every single image from Audubon’s titanic – life-sized, indeed – Birds of America (see here) is striking and beautiful. Here, the flamingo and Columbia jay provide a pleasing chromatic juxtaposition.

Richard Owen (1804–1892), On the Archetype and Homologies of the Vertebrate Skeleton, 1848; Homologies of the skeleton of an ostrich.

Richard Owen was one of the most important figures in nineteenth-century vertebrate morphology, and dedicated great effort to seeking the organizational scheme underlying the structure of backboned animals. All vertebrates are constructed from the same set of anatomical precursors, and Owen was fascinated by how these precursors could generate creatures as diverse as fish, turtles, birds and whales. Owen sought to extrapolate back from modern animal structure to a single common ancestor of all vertebrates with an ordered, regular body structure from which all its descendants’ body plans are derived. We now think that, while vertebrates do indeed share a single, long-lost ancestral species, its body plan was probably not as satisfyingly regular and neatly organized as Owen had hoped.

Louis Agassiz (1807–1873) and Augustus Gould (1805–1866), Principles of Zoölogy, 1851; Crust of the Earth as related to Zoölogy.

Like William Smith (see here) and George Lyell, the Swiss scientist Louis Agassiz (see here) studied how different rock strata contain, and may be distinguished by, animal fossils from particular epochs. However, this diagram, almost zodiacal in its regularity, oversimplifies the messy pattern of animal evolution in its attempt to make its point.

Anonymous, Brockhaus and Efron Encyclopedic Dictionary, 1890–1907; Fossil vertebrates.

Once a striking zoological illustration is created, it may have a life beyond its initial context. Many of the images in this artist’s resource are near-facsimiles of previously published designs, and many have been published repeatedly up to the present day.

Aristide-Michel Perrot (1793–1879), Tableau des Animaux et des Végétaux Éxistans Avant le Deluge (coloured print), 1844.

No doubt inspired by De la Beche’s Duria Antiquior (see here), Perrot’s Tableau veers towards the scientifically suspect in places. Elephants coexist with plesiosaurs, bats with pterosaurs, and an ammonite shell perches vertically on the crowded beach.

Johann Georg Heck (1823–1887), Iconographic Encyclopedia Of Science, Literature and Art, 1851, vol. 1, p. 74; The Order of Life: 144 images showing the progression and systematization of life.

Benjamin Waterhouse Hawkins (1807–1894), The Crystal Palace Dinosaurs, 1853–1855; Author’s photograph.

One of the most unusual urban sights, ‘The Crystal Palace Dinosaurs’, as they came to be known, were constructed near the site in south London where the ‘Crystal Palace’ centrepiece of the 1851 Great Exhibition had been relocated. A scientific illustrator, Hawkins sought the advice of eminent palaeontologists before commencing these first full-size recreations of ancient animals, yet much was left to his own interpretation – the texture of their skin, for example. Now often noted for its scientific inaccuracies, this much-loved diorama is really a testament to its creator’s success in representing the scientific knowledge of his time.

Josiah Nott

Polygenism, Race and Slavery

TIMELINE

1804: Josiah Nott born

1833: Establishes medical practice in Alabama

1854: Types of Mankind published

1871: Charles Darwin publishes The Descent of Man

1873: Josiah Nott dies

For years before Darwin and Wallace’s discovery of natural selection, scientists had pondered the origin of humankind. Most proponents of evolution believed, as biologists do today, that modern humans have a single ancestral origin, and thus are all related. Indeed, the fact that members of all ethnic groups can inter-breed to produce healthy, fertile children meant that, according to prevailing ideas, all humans are a single species. Yet for a brief period in the mid-nineteenth century, one person almost derailed this prevailing ‘monogenic’ theory of human origins.

A practising physician-surgeon in Mobile, Alabama, selective refuter of Christianity, and supporter of slavery, Josiah Nott dominated what was called the American School of Ethnology. Prevailing social attitudes meant questions of race and superiority were debated in a more heated atmosphere in the United States than in Europe. Slavery was ever-present and many sought justifications for its continuation.

Nott rejected evolution, and instead reverted to an almost bestiary-like form of animal classification, which he could then stretch to support his theories. For example, he was the most assertive proponent of ‘polygenism’, the idea that each of several ‘human races’ was a distinct entity separately created by God.

He disregarded scientific evidence and scriptural narrative with equal abandon, stating that mixed-race people were akin to mules, and that Genesis recounted only the history of a small subset of mankind descended from a white Caucasian Adam. As a result, Nott claimed the evidence showed the white man was justified in dominating the black man, whose attributes render him the perfect slave.

Despite Nott’s fervour, polygenism was short-lived. He could not deny that Darwin’s theories were the death-knell for a separate origin for human races, and ceased his research and writing. Soon, he could no longer tolerate living in what he called ‘Negroland’, and moved his practice to the relatively white and bible-less New York City.

However, polygenism, and the racial theories it supported, did not disappear entirely.

Josiah Nott (1804–1873), George Gliddon (1809–1857) and others, Types of Mankind, 1854; Apollo Belvedere, negro and chimpanzee.

Shocking today, the implication of this image is clear. In the decades when the mechanisms underlying the process of evolution remained unexplained, the large amounts of newly acquired biological information could be used to justify almost any conclusion. Here, the anatomy of a chimpanzee and black and Caucasian humans are unquestioningly compared, without reference to any evidence regarding their origins, adaptations or ecology.

Josiah Nott (1804–1873), George Gliddon (1809–1857) and others, Types of Mankind, 1854; ‘Tableau to accompany Dr Agassiz’s sketch’.

Animal classification in the support of social dogma. This chart accompanied an article by Louis Agassiz (see here) in Nott and Gliddon’s 1854 review of the ethnography, geography and origins of the human races. Chiming with Nott’s polygenic beliefs, each of eight columns presents the animal inhabitants of one of the world’s major geographical regions. Every region is depicted as being inhabited by a distinct, discrete type of human, just as it is home to its own species of carnivorous and grazing animals.

Heinrich Georg Bronn (1800–1862), Untersuchungen über die Entwickelungs-Gesetze der Organischen Welt wahrend der Bildungs-Zeit unserer Erd-Oberfläche, 1858 (above and below).

Published the year before On the Origin of Species (see here), and known to be in Darwin’s own collection, Bronn’s lengthy book was a wide-reaching survey of existing geological and zoological knowledge. It featured varied representations of past changes in the diversity of animal groups over time, including the ‘spindle diagram’ on this page. However, most important is the tiny tree tucked away in a corner of one of its later pages (see opposite). Unlike the tree produced by Eduard von Eichwald (see here), this image emphasizes the progressive diversifications of animal forms, with branches and sub-branches appearing at apparently irregular intervals. Bronn’s reclusive shrub can thus be said to be the first truly ‘modern’ evolutionary tree.

Charles Darwin (1809–1882), Notebook ‘B’ excerpt, c.1837–1838 (above); On the Origin of Species, 1859; Sole figure (below).

Although Darwin had long considered the possibility that the evolutionary process could be represented in tree-like form, he himself drew few trees – and indeed doubted that sufficient fossil evidence would ever be available for credible trees of actual animal groups to be created. The image above shows the now-famous ‘I think’ page of his notebooks from as far back as the 1830s, in which he uses a tree-form to illustrate how the pattern of evolutionary change and diversification creates species with differing degrees of relatedness. In contrast, the only figure in On the Origin of Species (see opposite) is an austere thing, certainly lacking the fluidity of the little tree (see here) of Heinrich Bronn from the previous year. Darwin mainly uses his diagram to explain divergence of sub-populations within a species, but later explains how the tree may be traced back to larger twigs and branches, eventually encompassing all living things.

Karl Gegenbaur (1826–1903), Grundzüge der Vergleichenden Anatomie, 1859; Skeleton of the manus (forefoot) of various mammals (above); Posterior portion of the venous system of the frog, alligator and bird (below); Side view of the skulls of an ostrich, a crocodile and a python (below); Differences in the arrangement of the ribs and transverse processes in the Teleostei [bony fish] (below).

Alvin Jewett Johnson (1827–1884), Johnson’s New Illustrated Family Atlas, 1860; Map of the World Showing the Geographical Distribution and Range of the Principal Members of the Animal Kingdom.

It was to be Alfred Russel Wallace (see here and here) who developed our modern view of organisms in the context of particular geographical regions, but this earlier image shows this was not an entirely new field of inquiry.

Philip Henry Gosse (1810–1888), A History of the British Sea-Anemones and Corals, 1860; Anemones.

Having worked in fisheries at home and abroad, the British marine biologist Philip Gosse was the inventor of the marine aquarium, where his subjects could be observed, and drawn, at leisure.

John Phillips (1800–1874), Life on the Earth: Its Origin and Succession, 1860; Distributions in latitude of several races of Mammalia.

John Phillips (1800–1874), Life on the Earth: Its Origin and Succession, 1860; A scheme of proportionate life for the Lower Paleozoic Strata (above); Successive systems of marine invertebral life (below). A continuous curve that corresponds to the numerical prevalence of life (bottom).

Phillips was an English geologist who meticulously compiled data from around the world to create the first global system of standardized geological epocha. To do this, he developed many novel forms of organization of extant and extinct animal species. On these pages he surveys the distribution of living species by latitude and the prevalence of fossils in ancient strata, and on subsequent pages considers the vagaries of the rise and fall of different animal groups over time.

John Gould (1804–1881), The Mammals of Australia, 1845–1863; Ornithorhynchus anatinus.

Over the years, the charming platypus has presented more challenges to taxonomists than any other species. An assemblage of superficially unrelated characteristics, it is now known to be an extremely specialized mammal that diverged from the main mammalian ‘stem’ very early. It suckles its young, which defines it as a mammal, but lays eggs, possibly reflecting the retention of an ancestral mammalian characteristic. Its webbed feet and flat tail have evolved convergently with waterfowl and beavers, respectively, but its bill is a unique electroreceptive organ used to seek its aquatic invertebrate prey. The platypus even has the distinction of being one of very few venomous mammals.

John Gould (1804–1881), The Mammals of Australia, 1845–1863; Thylacinus cynocephalus.

The thylacine or marsupial wolf is one of the most dramatic known examples of convergent evolution – the evolution of similar features by distantly related species that nonetheless have similar ecological niches. The body shape, skeleton and, especially, the skull of the thylacine are remarkably similar to those of dogs and wolves, despite its genealogical affinities with kangaroos and wombats. The story of this unique animal is, however, one of the saddest in all zoology. Aggressively hunted by white settlers, the thylacine was probably driven to extinction in the early twentieth century – although tantalizing reports still occur of survivors in remote Tasmanian forests.

Gregor Johann Mendel (1822–1884), Versuche über Pflanzenhybriden, 1865; Page from original manuscript.

A Moravian friar, Gregor Mendel seems an unlikely person to have instigated the modern genetic revolution. We will probably never know what drove the Abbot of St. Thomas’ Abbey in Brno to conduct the experiments in plant hybridization that demonstrated that inheritance of biological characteristics involves physical mechanisms that follow arithmetic laws. Although his 1865 ‘Experiments in Plant Hybridization’ was published only six years after On the Origin of Species, it was largely ignored for forty years. In consequence, scientists’ ability to organize the animal world according to genetics was delayed by decades.

August Schleicher (1821–1868), Die Darwinsche Theorie und die Sprachwissenschaft, 1863; Stammbaum for Indo-European.

Evolutionary biology has overlapped to a surprising extent with linguistics, and in this diagram the linguistician August Schleicher takes a Darwinian approach to the changes and diversification of the Indo-European languages of Europe and south and west Asia (Stammbaum means ‘family tree’). Of course, as well as splitting, languages can share elements and possibly even merge, something that is now thought in some ways to be true of animal species, too (see here). Also, it is possible that even the word ‘evolution’ was appropriated by eighteenth-century biologists from seventeenth-century studies of language change over time.

Alfred Frédol (1804–1863), Le Monde de la Mer, 1866; Développement d’un oiseau.

Frédol was the pseudonym of Alfred Moquin-Tandon, Professor of Zoology and Director of the Jardin des Plantes in Paris.

Richard Owen (1804–1892), On the Anatomy of Vertebrates, 1868; Upper views of the cerebral hemispheres of the human, a foetus and other primates.

A strange montage in which human cerebra are interspersed with those of other primates. The top row, from left to right, depicts Midas (tamarin), Callithrix (marmoset), Macacus (macaque) and a human infant, while the bottom row shows the brains of a human foetus, a lemur, Cebus (capuchin monkey) and chimpanzee. The adult human brain dominates the right side of the image.

Charles Darwin (1809–1882), The Descent of Man and Selection in Relation to Sex, 1871; Orchestia darwinii showing the differently constructed chelae of the two male forms.

Having established the theory of natural selection, Darwin extended his initial ideas considerably, and indeed established the conceptual framework on which much of modern evolutionary biology is based. He discussed the origin of life, the speed and pattern of evolution, the origins of humans, and sexual selection (a different mechanism of evolution distinct from natural selection). In this diagram, Darwin notes that evolution sometimes leads to males of different morphologies within the same species.

St. George Jackson Mivart (1827–1900), On The Genesis Of Species, 1871; The walking leaf insect.

Alfred Russel Wallace

Animals in space, as well as time

TIMELINE

1823: Alfred Russel Wallace born

1848–1852: Expedition to Brazil

1854–1862: Expedition to the Malay Archipelago

1858: Publishes, alongside Darwin, paper on natural selection

1913: Alfred Russel Wallace dies

Along with Darwin, the Welsh naturalist and explorer Alfred Russel Wallace was one of the two architects of the now-dominant theory of natural selection. However, he was also the scientist who gave zoological classification a geography, as well as a history.

Although he never enjoyed the financial security into which Darwin married, Wallace was able to finance a far more adventurous life. Inspired by stories of ‘new lands’, he participated in an expedition to uncharted regions of the Amazon in the late 1840s, as well as a career-defining, eight-year trip to what are now Malaysia and Indonesia.

One feverish night in 1858, Wallace suddenly realized how species evolve, and immediately wrote to his long-time correspondent, Charles Darwin, who he knew had been toying with similar concepts for years.

From that point onwards, it is difficult to separate the two men’s roles in the theory of natural selection. They corresponded regularly, supported each other’s ideas and published together in 1858, the year before the publication of On the Origin of Species. Wallace remained Darwin’s greatest supporter, entirely content to be the less famous of the two.

Alfred Russel Wallace (1823–1913), Darwinism, 1889; Variation of Icterus baltimore.

As well as his pivotal yet understated role in the theory of natural selection, Wallace was a founder of the science of biogeography – the study of the geographical distribution of animals past and present. Drawing on the work of the British zoologist Philip Sclater, Wallace identified the six global regions of faunal distribution still in use today (see here).

Indeed, the Oriental and Australian regions remain separated by the eponymous invisible ‘Wallace Line’, which he himself discerned partitioning the animal life of Borneo and Bali in the west and that of Sulawesi and Lombok in the east.

Alfred Russel Wallace (1823–1913), Darwinism, 1889; Variation in body, wing, tail and head size in forty male red-winged blackbirds (above); Mimicry among insects from the Philippines (below).

Alfred Russel Wallace (1823–1913), The Geographical Distribution of Animals, 1876; a Brazilian forest, with a range of native mammals.

Alfred Russel Wallace (1823–1913), The Geographical Distribution of Animals, 1876; The Palaearctic region (above); A Malayan forest, with its characteristic birds (below).

Harold Dalton (1829–1911); Microscope mount including insect scales and single-celled animals, c.1875 (above); Micromosaic (below).

Micromosaics enjoyed a brief flurry of popularity in the late nineteenth century. Insect scales, single-celled diatoms and other minutiae were painstakingly harvested, categorized and arranged on microscope slides, using needles and tiny air-blowers to create kaleidoscopic arrangements. Zoologists harvested zoological specimens from around the world to incorporate into these artistic mini-tableaux.

August Weismann (1834–1914), Studies in the Theory of Descent, 1882 (above and below).

Weismann developed the ‘germ plasm’ theory of inheritance, by which offspring inherit traits solely by means of information carried in the germ cells – eggs and sperm. Today, this concept is the main refutation of the mode of evolution proposed by Jean-Baptiste Lamarck (see here) – in which influences on any part of an animal’s body may be inherited by its offspring. Weismann’s own interpretations of his germ plasm theory were, in fact, more subtle and complex.

George Romanes (1848–1894), Mental Evolution in Animals, 1883; Frontispiece.

A friend and supporter of Darwin, Romanes was particularly interested in finding links between cognitive processes, and even consciousness, in animals and humans. At a time when the theory of natural selection was very new, and the brain remained poorly understood, this was a challenging topic in which to become embroiled.

Ernst Haeckel

Trees of Species, Trees of Races

TIMELINE

1834: Ernst Haeckel born

1862: Becomes Professor of Zoology at Jena

1866: Meets Charles Darwin; Generelle Morphologie der Organismen published

1904: Kunstformen der Natur published

1919: Ernst Haeckel dies

Professor of Zoology at Jena, Ernst Haeckel was a potent force in nineteenth-century zoology. An über-Darwinist, he took the theories of his more cautious English contemporary and created his own, more strident version – Darwinismus.

Originally an artist, and strongly influenced by the German Romanticists, Haeckel travelled the world collecting and painting the specimens that would support his theories of evolution and natural selection. He toured with a menagerie of skeletons and embryos, and his profusely illustrated books adorned the drawing-room tables of Europe.

For his time, Haeckel was unusually clear in his assertion that Man is just one species among many, and his zoological classifications are based on the idea that all life has evolved from a single, simple ancestor. Because of this, his evolutionary trees all have a single stem, and humans inhabit their rarefied crowns.

Haeckel wrote extensively on the origins of life in the simplest creatures – his ‘cell-souls and soul-cells’ – and developed uncompromising views on the progressive nature of evolution, and the links between evolution and the burgeoning field of embryology.

Ernst Haeckel (1834–1919), Natürliche Schöpfungsgeschichte, 1868; Die Familiengruppe der Katarrhinen.

To him, all progress towards perfection was the same, but he is now mainly remembered for over-extending his theories into religion and race. When viewing the beauty of his zoological diagrams, it can be hard to believe this is the same man who described Judaism as being intermediate between ‘primitive’ paganism and ‘advanced’ Christianity, or wrote that non-Europeans are ‘…physiologically nearer to the mammals – apes and dogs – than to the civilized European. We must, therefore, assign a totally different value to their lives.’

Although his writings were later lauded by the Nazis, Haeckel’s art remains a glorious testament to the classificatory traditions of nineteenth-century natural philosophers.

Ernst Haeckel (1834–1919), Kunstformen der Natur, 1904; Chiroptera – Fledetiere.

Ernst Haeckel (1834–1919), Anthropogenie oder Entwickelungsgeschichte des Menschen, 1874; Stammbaum des Menschen.

Few could devise evolutionary trees like Haeckel, and this single gnarled example – the ‘Pedigree of Man’ – summarizes the entirety of animal creation. In a way that would be unacceptable today, it also allocates animals to different ‘grades’ of advancement at different heights – single-celled animals, invertebrates, vertebrates, mammals – ascending towards the peak of creation which is, of course, humans.

Ernst Haeckel (1834–1919), Kunstformen der Natur, 1904; Discomedusae.

Ernst Haeckel (1834–1919), Kunstformen der Natur, 1904; Ostraciontes – Kofferfische.

Ernst Haeckel (1834–1919), Natürliche Schöpfungsgeschichte, 1868; Hypothetical sketch of the monophyletic origin and of the extension of the 12 Races of Man from Lemuria over the Earth.

Ernst Haeckel (1834–1919), Die Radiolarien, 1862.

Radiolarians are single-celled organisms that secrete complex and distinctive silica skeletons, and were Haeckel’s favourite artistic subjects.

Ernst Haeckel (1834–1919), Generelle Morphologie der Organismen, 1866.

This evolutionary tree, unusually, classifies the mammals according to the structure of their placenta – their most anatomically diverse organ.

Ernst Haeckel (1834–1919), Natürliche Schöpfungsgeschichte, 1868; Ascidia (sea squirt) and Amphioxus (lancelet).

Ernst Haeckel (1834–1919), Anthropogenie oder Entwickelungsgeschichte des Menschen, 1874; Pedigree of the Indo-Germanic languages.

Ernst Haeckel (1834–1919), Anthropogenie oder Entwickelungsgeschichte des Menschen, 1874; Systematic survey showing the derivation of the germ layers of the Amphioxus.

Ernst Haeckel (1834–1919), Gemeinverstaändliche Vortraäge und Abhandlungen aus dem Gebiete der Entwickelungslehre, 1902; Schema der Perigenesis (transmission of life-force between generations).

Albert Günther (1830–1914), Report on the Deep-Sea Fishes Collected by H.M.S. Challenger during the Years 1873–1876, 1887; Aphanopus carbo and Lepidopus tenuis at 315 fathoms.

The Challenger expedition was the first global research survey of the sea, and returned a wealth of new information, especially concerning the previously unknown deeps. Indeed, the expedition sounded the deepest part of the ocean at over 35,500 feet, now known as the Challenger Deep. A century later, an ill-fated space shuttle was also named after the boat.

Albert Günther (1830–1914), Report on the Deep-Sea Fishes Collected by H.M.S. Challenger during the Years 1873–1876, 1887; Thachyrhynchus murrayi at 555 fathoms, T. longirostris and T. trachyrhynchus.

Francis Maitland Balfour (1851–1882), The Works of Francis Maitland Balfour, 1885; Development of the yolk of Pristiurus (catshark).

Working in Cambridge, Balfour was instrumental in discovering and comparing developmental processes in both vertebrates and invertebrates, before his untimely death in a climbing accident in the Alps.

Adolphe Millot (1857–1921), Le Nouveau Larousse Illustré, 1898; Paléontologie.

A profusion of fossil forms is presented here, but with little organization. Ancient mammoths, horse, anteaters, armadillos, fishes, turtles and molluscs jostle for prominence, often positioned according to their ability to tesselate into the smallest possible area.

Charles Cornevin (1846–1897) and François-Xavier Lesbre (1858–1942), Traité De L’age Des Animaux Domestiques D’après Les Dents, 1894; Pince, mitoyenne et coin inférieurs d’un cheval de cinq ans et demi.

Animals’ teeth may, with varying degrees of accuracy, be used to determine their age. The age of eruption of teeth is indeed fairly consistent, but the reliability of assessing their subsequent wear is debated. Here the pattern of wear of the lower incisors of the horse is shown.

Jonathan Zenneck (1871–1959), Die Zeichnung der Boiden, 1898; Boa species from the genera Epicrates and Corallus.

Soon after compiling this treatise, Zenneck left biology altogether and was to make important advances in radio and cathode-ray technology.

Yves Delage (1854–1920) and Edgard Hérouard (1858–1932), Traité de Zoologie Concrète, Tome II: Mésozoaires et Spongiaires, 1898–1899; Arbre généalogique des spongiaires (above); Sponges (below).

The sea supports a wider range of animal forms than dry land, so the classificatory challenges of its inhabitants are greater, as is their apparent alienness. In fact, some of the creatures shown opposite are closely related to the vertebrates, and thus ourselves, while others are only very distant relatives.

Yves Delage (1854–1920) and Edgard Hérouard, Traité de Zoologie Concrète, Tome VIII: Les Procordés, 1898–1899; Balanoglossus (above); Clavelinidae (below).