Aristotle’s Ladder, Darwin’s Tree: The Evolution of Visual Metaphors for Biological Order

Before Darwin’s On the Origin of Species was published in 1859, evolutionary trees of life were a novelty; after Darwin, they were a necessity, not likely because of Darwin’s single tree-like diagram in this work but because of the foundations that he laid for “descent with modification by means of natural selection.” The trees that ensued did not, however, bloom equally in all areas dealing with evolutionary matters. As well, for almost the next one hundred years following the establishment of a pattern of visual representation soon after Darwin, with few exceptions, we see relatively little lasting change in how the visual portrayal of evolution affected our perceptions of the process and pattern of evolution. This does not mean that this long interval of time was devoid of tree-like representations—far from it; however, as an understanding of genetics and the importance of population-based studies emerged, these trees took on new meanings.

With the turn of the twentieth century, American scientists became, for several reasons, prominent cultivators in the production and dissemination of phylogenetic trees. The science of paleontology began ascending in stature in the United States, especially in East Coast institutions. This occurred in part because of the opening of the western United States, which began in the latter part of the nineteenth century. There an incredible wealth of fossils, notably of vertebrates, sparked interest in understanding their evolutionary past. Although still a harsh environment at the time, the American West afforded far easier access than any other parts of the world with comparable fossil riches. Additionally, even though fossil invertebrates were more common, rich troves of the newly recognized dinosaurs and the best record of horse evolution, for example, ensured that depictions of vertebrate evolutionary history would predominate; and they did for the next sixty years.

One of the earliest trees to appear after the publication of On the Origin of Species (1859) was one by Mivart (1865), dealing with relationships of primates using features of the axial skeleton (vertebral column) (figure 5.1A). In 1867, Mivart published another primate tree, this time based on the appendicular skeleton (fore- and hind limbs) (see figure 5.1B). As Mivart (1867) points out, in the latter tree he is only trying to “express the degrees of resemblance using the appendicular skeleton of Primates, not the affinities indicated by their osteology generally, still less that evidences by the totality of their organization” (424). Both papers provide some of the earliest attempts to demonstrate that different parts of the anatomy of organisms may yield different results in understanding evolutionary relationships and in the totality of evidence needed to best understand relationships, or what we today call total evidence. Mivart must be credited with laying out very clearly the anatomical basis for his trees, but claims that he placed humans as a lateral branch on his tree (a quite modern perspective), whereas others such as Haeckel placed humans at “the apex or culmination of evolution,” are incorrect (Bigoni and Barsanti 2011:6). As we will see, Haeckel, like Mivart, placed humans on different parts of their trees, depending on what was intended. Also, recall from chapter 4 that Mivart’s trees likely influenced Darwin’s attempts at a similarly stick-like tree that he sketched in 1868 but never published (see figure 4.15).

Mivart stands as a somewhat tragic scientific figure. Although an early acolyte of Darwin’s “descent with modifications by means of natural selection,” he soon turned against Darwin’s ideas. In On the Genesis of Species (1871), although not totally rejecting natural selection, he notes that whether the theory of natural selection “be true or false, all lovers of natural science should acknowledge a deep debt of gratitude to Messrs. Darwin and [Alfred Russel] Wallace, on account of its practical utility. But utility of theory by no means implies its truth” (22). After this backhanded compliment, Mivart then compares Darwin’s and Wallace’s contributions with those of alchemists dealing with light emission and the atomic theories. Later he provides a list of “doubts and difficulties” with natural selection (34). Although antiselectionist, his continued argumentation in favor of evolution ran him afoul of the Catholic Church, of which he was a devote member. Both camps shunned him.

A more obscure but nevertheless interesting tree was produced by another early Darwin supporter, the German paleontologist Hilgendorf, who published in 1866 and 1867 what may have been the earliest evolutionary tree based on fossils—in this case, fossil snails from the Middle Miocene (about 14 million years ago) of the Steinheim Basin in Germany. His 1867 tree occurs as a small image in the middle of a large figure surrounded by the images of the snails he studied (figure 5.2A). Hilgendorf’s publications in 1866 and 1867 arose from his dissertation of 1863, but this apparently did not include a tree illustration (Rasser 2006). Darwin knew of Hilgendorf, a young man in his twenties when he completed his dissertation. Beginning with the fifth edition of On the Origin of Species (1869), Darwin writes: “Hilgendorf has described a most curious case of ten graduated forms of Planorbis multiformis in the successive beds of a fresh-water formation in Switzerland [sic]” as a case showing “intermediary forms” (362). Darwin does not indicate here or in later correspondence with various people anything specifically about Hilgendorf’s trees, other than noting the “graduated forms” within “successive beds” that possibly allude to these trees of Planorbis multifromis (now Gyraulus kleini). Although paleontologists previously presented successions of fossils based on their fieldwork, Hilgendorf, armed with the new Darwinian theorizing, argued that this succession resulted from one species evolving into another. More recent studies show Hilgendorf’s assessment of in situ evolution from one to three founder species within the Steinheim Basin to be more right than wrong (Nützel and Bandel 1993).

This idea that one form can give rise to another within a given area was controversial in Hilgendorf’s day and remains so in some biological quarters. Not until more than ninety years later did this concept of evolutionary change become crystallized as anagenesis, or the change of a single population or species in one area without splitting to form two or more daughter species. The splitting of a single species or lineage to form two or more daughter species represents cladogenesis, or the forming of new clades. Although he did not originate the terms, George Gaylord Simpson (1961) clarified them as used today. What Hilgendorf (1867) was showing in his diagram (see figure 5.2A) still represents a good example of anagenesis—in this case, in an impact crater in Steinheim Basin, Germany, in which a founder species of freshwater snail gives rise over time to a single descendant lineage of species, as well as giving rise cladogenetically to several other species (Nützel and Bandel 1993).

Gaudry (1866), one of a few French scientists who early on embraced Darwin’s ideas (Tassy 2006), like Hilgendorf produced a series of paleontological trees showing anagenesis and cladogenesis in his monograph on the mammals of Pikermi, Attica, Greece. Gaudry and colleagues had worked these beds for many years, recovering thousands of bones that at the time represented thirty-five mammal genera, but Gaudry was not “content with carefully describing all these interesting forms, as were his contemporaries. In pointing out the differences, which separated them from known forms, he was led to consider the resemblances that they showed to other extinct forms or to living forms. He sought the bonds, the relationships, which united the ancient organisms to one another and to living forms” (Glangeaud 1910:422).

In this slim, sixty-eight-page monograph, Gaudry manages to present us with five quite detailed, geologically based phylogenies of hyenas, elephants and their relatives, rhinoceroses, horses and their relatives, and pigs and peccaries with their presumed ancestors and covers some 50 million years of mammalian evolution for all continents except Australia and Antarctica—quite an undertaking. Unlike Hilgendorf, who restricted himself to arguing for the evolutionary replacement and minor splitting of one lineage of freshwater snails in a lake system in Germany, Gaudry attempted something far more grandiose: the unraveling of the evolutionary history of five groups of mammals over great time and vast geography. Today, many phylogenetic studies based on morphology of living and extinct species, as well as molecular data, incorporate a wide a variety of species both in time and in space, as did Gaudry. Nevertheless, it now is quite uncommon to make such sweeping claims of ancestral–descent relationships within an anagenetic series of the kind drawn by Gaudry, whereas the much more geographically and usually more time-restricted kind of studies done by Hilgendorf are found today. It must be emphasized that neither of these studies harkens back to the ladders or scala naturae found in the works of Charles Bonnet or even in Jean-Baptiste Lamarck’s earlier endeavors, which imagined a step-like or even seamless chain of being. Rather, both Hilgendorf’s limited and Gaudry’s expansive studies are attempts at unraveling what these authors hypothesized as very specific ancestor–descendant relationships. Gaudry’s tree for horse evolution appears in figure 5.2B. As discussed in chapter 1, phylogenies of horses as well as that for groups such as elephants and their relatives served as part of the backdrop for the first half of the twentieth century as to how evolution occurs and how it should be drawn.

The fourth and youngest of these aspiring scientists was the Russian paleontologist Kovalevskii, who translated into Russian the works of a number of well-known European scientists, including some of the works of Darwin. In search of a research topic to further the Darwinian view of evolution, Kovalevskii was influenced by Thomas Henry Huxley’s interest in the evolution of horses and other ungulate, or hooved, mammals. Kovalevskii’s (1876) major work in this area resulted in a well-argued, three-part monograph completed in late 1873 through early 1874 that Gaudry highly praised (Vucinich 1889). Figure 5.2C shows Kovalevskii’s quite rectilinear trees, the style of which became popular in the 1970s to indicate Niles Eldredge and Stephen Jay Gould’s (1972) punctuated equilibrium hypothesis of evolutionary rates, which emphasized rapid speciation followed by stasis. Even if he appears prescient, this was not Kovalevskii’s intent; it simply was his manner of drawing. The scale is too small to make out details, but on the left the three vertical lines show his anagenetic evolutionary view for tapirs, horses, and rhinos (what today we call perissodactyls, or odd-toed ungulates). The most complex tree, in the middle, is for cattle, sheep, antelope, and the like (what today we call ruminant artiodactyls, or even-toed cud chewers). On the far right, the tree is for true pigs and peccaries, or suiforms. The hippopotamus is shown as a dot at the top right but is not connected to the suiform tree. It is now known to be the nearest living relative to all cetaceans and not closely related to suiforms. There is an abbreviated geological scale on the left starting back in the Cretaceous period, over 65 million years ago, and ending at the present day. At the bottom middle is a large dot for the imagined Urungulata, or protohooved ancestor. Interestingly, we now know with some certainty that the earliest known ungulate, aptly named Protungulatum, is indeed a rare occurrence in the Cretaceous of North America (Archibald et al. 2011). Of considerable interest in Kovalevskii’s figure is what he does not attempt to show; he leaves many taxa as dots not connected to other dots. He does group them in a general way with the other taxa that he believes they might be nearest to evolutionarily, but he sounds a cautionary note about what he thinks we do not know in leaving dots unconnected. If only some modern systematists were so cautious in drawing relationships.

Haeckel, the last of the young Turks discussed here, early on set out to test and promote Darwin’s theories. A brilliant and controversial German biologist, Haeckel earned the sobriquet “German Darwin.” He produced a veritable thicket of quite different sorts of trees and in so doing dominated this early phase of the gilded age of evolutionary trees. Haeckel coined a number of the biological terms still used today, the most germane for this discussion being “ontogeny” for individual organismic development and “phylogeny” for evolutionary development—hence the now common use of phrases such as “phylogenetic tree” (of obvious meaning) and “phylogenetic systematics” for the study of the evolutionary relationships of organisms.

In 1866, in the second volume of his two-volume work Generelle Morphologie der Organismen (General Morphology of Organisms), which, unfortunately, was not widely read or translated, Haeckel prepared no fewer than eight phylogenetic trees. His first tree addresses adroitly one of the problems that vexed not only Darwin but also most biologists in the 1860s, and in fact still does: How did life begin, and how many times did it do so? Recall that Lamarck wrote of multiple origins and that Carl Edward von Eichwald’s many-trunked tree indicated the same idea (see figure 3.3). The translated title of Haeckel’s tree in the lower-right corner reads, “Monophyletic Family (or Genealogical) Tree of Organisms” (figure 5.3). In this volume, Haeckel also coined the word “Moneren” for what we term informally today as monerans for simple, single-celled organisms, but which we no longer use in formal taxonomy.

The lower-left corner in figure 5.3 lays out his three hypotheses. Starting with the lowest, “III, box: pstq (1 branch),” bounded by the letters p, s, t, and q, this hypothesis shows a single or monophyletic origin of all life from what Haeckel calls in Latinized phrasing “a single self-seeding common root of organisms.” From the single branch, left to right are the kingdoms “Plantae,” “Protista,” and “Animalia.” The second hypothesis, “II, box: pxyq (3 branches),” argues that plants, protistans, and animals arose separately. Finally, hypothesis 3, “I, box: pmnq (19 branches),” argues that six plant, eight prostistan, and five animal groups or lineages originated separately. These nineteen branches are labeled throughout the tree as well as along the mn line that crosses the figure.

With these three hypotheses from Haeckel’s first tree in Generelle Morphologie, he hedged his bets on the origin of life: a single moneran gave rise to all life; three different monerans originated and gave rise to plants, other monerans, and animals; or multiple lineages arose from of many original monerans (Richards 2008). In this work Haeckel, vacillates on which hypothesis he supports, but possibly because Darwin favored the third hypothesis in On the Origin of Species, Haeckel did as well.

Theodore Pietsch (2012) reproduces in considerable detail this and the remaining seven trees in Haeckel’s Generelle Morphologie in a visual compendium of trees of life, but the latter seven are reproduced in figure 5.4 at a much-reduced scale to simply convey their general form. The Roman numerals indicate Haeckel’s numbering scheme. All eight trees present the same general appearance of wispy, somewhat gnarled, almost vine-like forms with multiple or thin main stems. In a number of instances, the relationships shown no longer pertain, but as noted, it is their form that is of interest here. All seven trees show relative branchings and hence relations of one group to another. All seven trees also include extinct forms known only as fossils, and, as far as I can determine, none are shown as ancestors but rather as branches on the tree. When a name appears across a single major branch, the intent is to indicate what Haeckel perceives as the relative grade of evolution on the tree at that point in relative evolutionary time. Most such taxa then repeat across a number of smaller branches farther along the tree or appear terminally, with brackets superintending the groups they include. For example, in the uppermost left phylogeny of plants in figure 5.4, Gymnospermae is found both as a grade (lower arrow), leading to both gymnosperms and angiosperms, and as a higher taxon written across extant branches (upper arrow). This explains Haeckel’s reference to more primitive and more advanced grades in evolutionary history—shades of Aristotelian ladders along with his tree motif.

Five of the seven trees have an additional tree in a lower corner showing the same relationships portrayed on the larger tree, but with fewer names to help the reader understand higher-level relationships without the clutter of additional smaller group names. The one significant difference is that the trees for echinoderms and vertebrates are, as Haeckel notes, “paleontologically based,” meaning that he has provided a geological scale on the left side of each of these figures into which extinct forms and the origin of groups are placed at the appropriate geological interval, but again ancestors are not identified. In all eight figures and all the included trees, he shows us the relative branching of taxa, but not any ancestor–descendant relationships.

The importance of real or hypothetical ancestors comes with Haeckel’s next major set of trees beginning with Natürliche Schöpfungsgeschichte (The History of Creation, 1868). He based this work on a series of public lectures on the theory of evolution and how it applies to human origins, which helped bring Haeckel to prominence as one of the major proponents of evolutionary theory at the time. Unlike Generelle Morphologie der Organismen (1866), this newer work was more widely read and translated, hence his broader renown. In it, he provides us with fourteen figures that he identifies as Stammbaum (Stammbäume), a family or genealogical tree(s). One of these, labeled “Ahnenreihe des menschlichen Stammbaums” (Line of Ancestors of the Human Family Tree), provides no tree, but two columns. The two left columns indicate geologic time. Translations of the two right columns are “Animal Stages of Human Ancestors,” indicating relatively higher taxonomic groups, and “Living Closest Ancestral Stages,” indicating supposed living examples of the respective groups—a ladder-like array showing relative order of appearance or ascendance during geologic time, ending at the top with peoples whom Haeckel and many others at the time regarded as primitive humans.

Of the other thirteen at least partially branching diagrams, four forms are represented: five trees use a bracketed architecture showing more inclusive groups as one moves up the tree; two have geologic time scales and are even wispier versions of those in Generelle Morphologie, resembling interconnected strands of upside-down feathery Spanish moss; five have broom-like tufts, one emanating from the next lower; and one has a geologic time scale but in very severe rectilinear form. Figures 5.5 and 5.6 show examples of these four kinds of tree, with more examples available in Pietsch’s (2012) tree diagram compendium.

Three of the five trees with the bracketed architecture examine ideas on the polyphyletic, or multiple-origin, hypotheses of life showing various amounts of branching. The other two, decidedly more branching, detail the evolutionary history of hooved mammals in one and that of primates in the other. The diagrams also include some real and imagined ancestral forms. Figure 5.5A is one of the kinds of these bracketing diagrams—in this case, dealing with the origin of plants. At the base of the diagram Haeckel indicates “numerous vegetable monerans, created independently by spontaneous generation,” leading separately upward to ferns, lichens, and so on, as well as to other groups that did not lead anywhere, as indicated by the question marks. In the middle, he shows a ladder-like ascent starting with the Archephyta (ur-plant) near the bottom up to the angiosperms (Decksemige) at the top. Haeckel notes in the explanation of this figure that in plate II, he shows the alternative view of plant origins as monophyletic (see figure 5.5B). The plant taxa shown along the top are essentially the same as those shown in figure 5.5A, and along the left is a geologic time scale, so that here we see Haeckel’s view as to when various major groups of plants arose. Excepting some question marks on the figure, no text appears on the phylogeny. Figure 5.5B, along with a similar one for animals, represents among the most finely grained branching diagrams ever produced. The number of branchings at any given geologic interval varies considerably, so the relative density represents how speciose any given branch is at any given time. For example, Haeckel speculates (indicated by question marks) that angiosperms appear before the Triassic period, but the increase in density of branchings shows a great increase in species numbers beginning in the Cretaceous period. The small inset tree in figure 5.5B is reminiscent of Haeckel’s trees from 1866 in its wispy, vine-like form, which shows simplified, higher-level groupings of plants.

Figure 5.6A shows one of Haeckel’s (1868) five broom-like tuft diagrams, chosen for representation here because it repeats in a slightly different form the “Family Tree of Organisms” from Generelle Morphologie (1866) (see figure 5.3), which also addressed the issue of the poly- versus monophyletic origin of life. Beyond the obvious differences in the wispy, vine-like versus the broom-like tufts, the figures are similar and different in some other important aspects. They are similar in showing the three versions of monophyletic, triphyletic, and polyphyletic origin of life—even using the same letters to categorize each level. They are different in several ways, all intended to pander to a wider audience—recall that Natürliche Schöpfungsgeschichte (1868) sprang from popular lectures and was far more widely read and translated than Generelle Morphologie (1866). This shows in his trees dealing with the origin of life. In the figure from 1866, he is first and foremost concerned, after the question of monophyly versus polyphyly, with showing the branching patterns of the main groups within plants, prostistans, and animals, identifying them with Latinized names. In the figure from 1868, he adds German words as well as the Latinized words for public consumption, but more important is the concept of ancestry. He indicates an “Urstamm,” or archetypal stem, for plants, protistans, and animals. Above this are noted “Ur-plants,” “Ur-beings,” and “Ur-animals.” Above this, we find some branching, but the idea of a ladder-like form has taken hold. Especially compare the “plants” on the left sides in figures 5.3 and 5.6. In the former, the relationships are largely of a branching nature, whereas in the latter, they for the most part form a ladder: green algae, moss, ferns, Gymnosperms, angiosperms. The idea of an “ur-,” or archetypal, organism comes from the antecedent of science, natural philosophy, as embodied in Goethe’s idea of “ur-phenomenon” (Urphänomen), which Haeckel cannot quite escape (Seamon 1998). His Darwinian side pushed for the branching of life, whereas his intellectual upbringing in natural philosophy pulled him toward Goethe’s ideas of the “Ur-” that still enticed the general populace, helping to create the popularity of his 1868 book.

As a coda to his trees in Natürliche Schöpfungsgeschichte, I show Haeckel’s single severe, rectilinear tree (see figure 5.6B). Although produced in the 1860s, its design would have been equally comfortable in the 1920s and 1930s Bauhaus tradition. It again identifies “Ur-” taxa, here leading to six major lineages of animals placed within a geologic time scale. One could read what we today term parallel evolution within a group because of the nearly parallel lines found in his trapezoidal shapes, but this would be hyperbole. In the caption, he writes that one should read the text for explanation, but reading the text tells us nothing of why he constructed the phylogeny in the manner he did. It remains a mystery, other than suggesting that between his 1866 work, in which all the figures had a similar form, and his 1868 work, for which he gained much more popular acclaim, including as a consummate artist, he decided to play with different phylogenetic forms. The only real scientific change was the admittance of ancestors into the phylogenetic fold—not a new idea, but one that seems to have haunted Haeckel’s later work, especially pertaining to human evolution and his views on human races (and even different human species [see chapter 8]). In later editions of Natürliche Schöpfungsgeschichte, in both the German (1873) and the English and American editions (1876), Haeckel used a wispier derivative of a very similar tree. This reinforced the idea that he simply was playing with design and representation in this tree; the severe rectilinear form disappeared.

Haeckel must be given credit for another innovation in his use of tree-like figures that first appeared in Natürliche Schöpfungsgeschichte, but not until the second edition, published in 1870. Scientists such as Alexander von Humboldt (1817) had produced altitudinal zonations of vegetation; others such as Théodore Lacordaire (1839–1840) had drawn up tables indicating the global distributions of various insects (Browne 1983); and still others such as Alfred Russel Wallace (1876) deserve credit for helping to establish the science of biogeography, which deals with the geographic distribution of plants and animals. In the second edition of Natürliche Schöpfungsgeschichte, Haeckel published what appears to be the first map on which is superimposed a tree-like form, albeit a very complex form, that spreads over its surface and shows the origin and radiation of a group—in this case, of humans (figure 5.7). Although Haeckel had no knowledge of genetics, clearly his contribution is an antecedent of today’s study of phylogeography, which concerns the biogeographic spread and distribution of populations, mostly using genetic data. He places the origin of humans on a hypothetical continent in the Indian Ocean. Named Lemuria (presumably after lemur, Latin for “ghost of the departed”) in 1864 by the English zoologist Philip Sclater, it supposedly connected various regions in the Indian Ocean, including Madagascar, home to real lemurs. Haeckel explains in the text that Lemuria subsequently disappeared below the waves. Although quite fanciful sounding to us, the idea of lost continents as stepping-stones for the spread of plants and animals was not an uncommon notion until the theory of continental drift took hold after the middle of the twentieth century, rendering the need for such imagined lost continents generally unnecessary.

The final work of Haeckel discussed here, Anthropogenie; oder, Entwickelungsgeschichte des Menschen (Anthropogeny; or, Evolutionary History of Man, 1874), like Natürliche Schöpfungsgeschichte (1868), proved popular enough to be translated into English. In it, he presents two “genealogical trees” (Stammbäume), one for vertebrates (Wirbeltheire) and one for mammals (Säugethiere), each of which is accompanied by a classification that Haeckel calls “Uebersicht über das phylogenetische System” (Overview of the Phylogenetic System) for each particular group. Figure 5.8A shows the phylogenetic classification of mammals, and figure 5.8B displays the accompanying “Genealogical Tree of Mammals.” To my knowledge, these are the first classifications identified as phylogenetic, but this is not surprising inasmuch as Haeckel coined the term “phylogeny.”

Today, for a biologist doing systematics, the modifier “phylogenetic” indicates a one-to-one correlation between the classification and the phylogeny, such that one can produce the classification based on the phylogeny and vice versa. Such is not the case for Haeckel’s trees and classifications, because in his trees, Haeckel shows not only branching of various groups, which in principle can be indicated in the classification, but also progression from what he perceives as more primitive to more advanced groups. The classification thus cannot be constructed from the tree topology and thus in today’s usage would not be considered phylogenetic, as it does not reflect the evolutionary history of the group. For example, on the left side of the tree we see ungulates (hooved mammals) giving rise to cetaceans (whales and relatives) (see figure 5.8B), whereas in the classification, they are coeval groups (see figure 5.8A). Arguably, truly phylogenetic classifications along with their accompanying trees did not appear with any frequency until almost one hundred years later, but again it was a German who developed these newer schemes of discerning evolutionary relationships.

This brings us to Haeckel’s most famous tree in Anthropogenie, which in some ways represents the granddaddy of all phylogenetic trees, also combining progress with branching evolution (figure 5.9). This tree is titled “Stammbaum des Menschen” (Genealogical Tree of Humans). This is a massive specimen, so much so that it sometimes is referred to as Haeckel’s “great oak” (Pietsch 2012). Its gnarled, impressive form would make Edward Gorey proud. Read simply, it shows the ascendancy from the primordial moneran to the pinnacle of evolution—“Menschen,” or humans. Insects, which we today surmise number in the tens of millions of species, are a small side branch on the left of the oak. Haeckel did not know the true magnitude of this number, but I have little doubt that he realized that insect species far outnumbered mammal species.

Ink has been spilled interpreting this tree, arguing whether such a more ladder-like representation was somehow non-Darwinian (Bowler 1988; Dayrat 2003) or was simply attempting to show human evolutionary history from one perspective, whereas Haeckel’s other trees show humans as a mere branch on the tree of life (Richards 2008). I am inclined to the latter view, especially because as shown, Haeckel varied how he represented the evolution of other groups, as twigs or as central trunk. This said, Haeckel’s oak is different not merely in its massive appearance but also in its intended or unintended representation. Like Darwin, Haeckel was disinclined to reach for teleological, final, deity-based causes; yet like Darwin, Haeckel did see progress in evolution. Thus later species were more evolved or higher on the scale of life because of natural selection (Richards 2008). Unlike on the trees noted earlier on which higher taxa appear twice, first as a grade (or level organization) and then defining a clade (or branch) (for example, Gymnospermae in figure 5.4), in Haeckel’s oak higher groups appear generally only as grades where they fall along the progress toward humans. Thus this tree is certainly progressive, yet need not be teleological. Most likely, there is no ulterior motive other than Haeckel, as a brilliant scientist and equally talented artist, playing to an audience who could handle Darwin’s evolution as long as they themselves crowned the tree of life. Humans and especially Europeans were not yet prepared to see themselves as just one more evolutionary twig. Haeckel advanced this view with blatantly racist ideas of human evolution and dispersal.

The most ingenious, innovative, and complex yet largely forgotten trees produced at the time belong to Max Fürbinger (1846–1920). Fürbinger’s formal studies began in 1865 at Jena, Germany, and he completed his dissertation in Berlin on the skeleton and musculature of lizards with reduced limbs (Die Knochen und Muskeln der Extremitäten bei den schlangenähnlichen Sauriern, 1870). Although he worked widely in vertebrates, his research centered on reptiles and birds, and in Untersuchungen zur Morphologie und Systematik der Vögel, zugleich ein Beitrag zur Anatomie der Stütz- und Bewegungsorgane (Studies on the Morphology and Systematics of Birds, also a Contribution to the Anatomy of the Locomotor Organs, 1888), we find the quite amazing phylogenetic trees of extinct and living birds. The figures include two very wispy trees titled “Experimental Family Trees of Birds” and three “Horizontal (Planimetric) Projections of the Family Trees of Birds.”

What at first glance appear to be mirror-image trees actually show views of the same tree from either side, with various groups of birds emphasized from each view (figure 5.10A and B). The arrows indicate lines that Fürbinger drew across the trees, dividing them into lower, middle, and upper zones (figure 5.10C–E), which are shown in the three horizontal projections. These do not show exact slices through the tree at one given level but compress top–down or bottom–up views of the tree within each of the three zones, with distances between circles suggesting the degree of divergence. The circles and enclosing outlines are named for the relevant bird groups and the number of species in the particular groups. The size of a circle also indicates the relative number of species in that group.