I first read Charles Darwin’s On the Origin of Species in college, 130 years after its publication. By then, the book had long stirred controversy and secured its position among the most important scientific books ever published. From the moment his theory appeared, Darwin’s game-changing vision of how organic life on earth developed had an immense impact on myriad aspects of human culture—from philosophy and religion to literature and the history of art. Reading Origin for the first time, it wasn’t hard to see why. A fledgling humanist, I recall being struck by the efficiency and elegance of Darwin’s prose and by the book’s mix of concrete detail (pigeon breeding, instinctual behavior in ants and bees, animal life on islands) and grand theory. But I do not remember noticing the book’s one illustration, a diagram often referred to as the “Tree of Life,” lithographed by the London-based printmaker William West and inserted as a double-page fold-out towards the end of chapter 4, the chapter devoted to Darwin’s theory of natural selection. A minimalist array of upward-branching dashed and dotted lines annotated by letters and numbers and bisected by solid horizontal lines, it resembles the silken threads spun by a spider or the delicate craquelure of an antique vase.

I read Origin for the second time almost two decades later, as a scholar of art history and visual culture, studying works of art and other kinds of images in Europe and the Americas from the eighteenth century to the present. Much of my research considers how art-making and scientific inquiry intersected in this period—how knowledge in the sciences shaped the ways artists and their audiences envisioned their world, and how scientists used images to generate and communicate novel ideas and theories. This time around, I noticed Darwin’s diagram and paid close attention. A more careful reader in my forties than I was in my twenties, I understood that Darwin’s explanation of the diagram elucidates one of the most important principles of his theory: divergence of character. If natural selection describes how species originate, diverge, and become extinct—thereby accounting for the history and diversity of organic life—divergence of character explains, first, how small differences among varieties of organisms in a group can eventually lead to the existence of new species and, second, the critical importance of significant or extreme variation for those species’ survival. The finches of the Galápagos Islands most famously demonstrate divergence of character, even if they did not initially inspire the idea. Closely related but distinct species, the finches are differentiated by the size and depth of their beaks. These differences arose through the selection of specific traits that decreased competition among the birds and enabled many varieties of finch to coexist. Because each variety of bird consumed a different type of seed, as befitting the form of its beak, there was enough food to go around.

Darwin introduced the section on divergence of character in chapter 4 by announcing that his diagram would aid the reader “in understanding this rather perplexing subject.” On the face of it, however, the diagram hardly appears up to the task. The tree has an elegant, even fragile beauty that does not appear commensurate with the weight of Darwin’s thought. Nor does the diagram look much like other scientific illustrations of the period, which frequently referred directly to the observable world, either through visual representations of natural specimens or, in the case of charts and diagrams, robust verbal annotation. Darwin’s diagram, hypothetical rather than empirical, strips away all references to the real and cuts to the chase. The spare configuration that results is charged with demonstrating the immensely complex operations of divergence of character, but it is so abstract that it appears inconsequential, even superfluous. No wonder I missed it on the first pass.

Billed as an aid to understanding a “perplexing” subject, it is not immediately clear just what Darwin’s diagram does to augment and illuminate what he painstakingly explains in the text. His description of divergence of character runs for 16 dense pages and necessitates constant toggling between the fold-out image and text, a lengthy and disjointed process that runs the risk of obscuring the diagram’s meaning rather than revealing it. The book’s only illustration, the “Tree of Life” can feel extraneous, if not illegible. Even the dotted lines, a conventional means in nineteenth-century scientific illustration for indicating the hypothetical, make the diagram seem especially flimsy. Normally, dotted lines in a scientific image accompanied empirical information, as in the hypothesized anatomy of a dinosaur known through partial fossil remains. In Darwin’s diagram, however, nothing exists to anchor his theory to terra firma. The diagram is commonly called the “Tree of Life,” but it does not really look like a tree and Darwin never characterized it as such, nor did he fully embrace the tree metaphor for the mechanisms of natural selection, despite his poetic description of an endlessly branching tree at the close of chapter 4.

A case might be made for abstraction, which Darwin used before it became par for the course for scientific visualization in the twentieth century. Darwin wrote Origin after drafting a lengthy manuscript, titled Natural Selection, that offered a thorough elucidation of his theory, which he abandoned in 1858 after learning that the British naturalist Alfred Russel Wallace had independently arrived at many of the same conclusions. Origin was written quickly in the aftermath, as an “abstract” (Darwin’s own term) of the whole. Perhaps the diagram itself served as an abstract, this one illustration presenting the overall gist of Darwin’s theory in visual form, its expedience matching that of his swiftly written book.

Even so, the diagram undertakes an impossible task—impossible because evolution as Darwin conceived it presented a near-insurmountable problem for representation in the nineteenth century. Science in Darwin’s time gave rise to radically new ideas about existence, and this new knowledge created something like a crisis of picturing, with tried-and-true methods of visualization, conventionally mimetic in nature, coming up short. Time presented a special problem for scientific images. Just as the Copernican Revolution had displaced Earth from the center of the universe and Darwin’s own theory of evolution would go on to remake the human as just another animal among countless other beasts, geology and the closely related disciplines of paleontology and archaeology radically expanded the timescale of the earth while drastically reducing humanity’s share in its chronology. Deep time, in turn, introduced a very big problem of visibility. “The mind cannot possibly grasp,” Darwin wrote in Origin, “the full meaning of the term a hundred million years; it cannot add up and perceive the full effects of many slight variations, accumulated during an almost infinite number of generations.” What is more, “we see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.” So many tiny changes occurred over so much time, Darwin says here, that no mortal could begin to isolate and identify them, let alone capture them in an image.

In the Origin, Darwin never hesitated to point out gaps in science and in his knowledge—or as he put it, forms of understanding one could only “dimly forsee.” One gets the sense that, in the aftermath of a series of paradigm shifts in the sciences and in the wake of his own groundbreaking discoveries, he felt he knew even less about existence than before. Or, more accurately, that what he had learned about existence was monumental, even as it raised innumerable new questions and brought to light vast tracts of unexplored scientific terrain—what he referred to in the conclusion of Origin as a “grand and almost untrodden field of inquiry.” Perhaps Darwin’s diagram, a sparse and tenuous graphic array, stood in stark contrast to other scientific imagery in the period because he wished to resist implying that he had all the answers (not least because the book’s nearly five hundred pages of detailed explication and data, along with the overall confidence of Darwin’s rhetoric, could easily have suggested to the reader that Darwin believed he had it all in the bag).

Given this, I cannot help but think that Darwin’s diagram bears the burden of Darwin’s doubt. It begins in thin air, with a collection of unanchored, dangling lines. And it ends, at the top of the fold-out page, with a spare collection of lines, dashes, letters, and numbers. The spaces between each of the dots in the branching lines indicate a temporal interval. But as a series of empty spaces or blanks, they also conjure absence, as in the absence of seeing or knowing.

Even Darwin’s detailed, multi-page explanation of the diagram stumbles in places, registering notes of misgiving. Each of the intervals between the diagram’s horizontal lines, Darwin noted, represent a thousand generations, but “it would have been better if each had represented ten thousand generations.” Here, Darwin pauses amid his explanation to undercut his own design, signaling to the reader that all is not said and done, and that more knowledge—and a better diagram for articulating this knowledge—are yet to come. He does something similar on the following page. “I must here remark,” he wrote, “that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular.” A few sentences later: “In our diagram, the line of succession is broken at regular intervals by small numbered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere.” As Darwin made clear in Origin, natural selection was unpredictable and followed no strict script, differing according to geography and climate, arising from any number of causes, happening at vastly different rates, and yielding inconsistently and variably differentiated new species. Only a diagram simple and pliable enough to mean many different things could accommodate such variety and range.

Darwin’s self-conscious hedges suggest that he understood the diagram’s opacity, and wished to ensure the reader knew he was aware of this trouble. In this way, the diagram offers to the reader a kind of pedagogy, a primer on how to encounter the knowledge presented in Darwin’s book. With excitement, of course, and the feeling of having been convinced by Darwin’s claims, but also with a real sense of what was left to know, and what, in the end, might always exceed human grasp. That said, the diagram also succeeds, fantastically so, by wiping the slate clean, dismantling and destroying convention to clear a path for new modes of seeing and knowing and by generating, through abstraction, the possibility of an elastic and polyphonic mode of scientific visualization. In this way, Darwin tells us something about the history of life on our planet, but he also has something profound to say about the exertions of science and its verbal and visual languages, including the questionable capacity of any expressive form to delineate the nature of existence. ♦