At the end of January, 1859, some nine months prior to the publication of Darwin’s Origin of Species, Ernst Haeckel set out for Italy, where he would undertake his habilitation study. He wished to do something in marine invertebrate biology, but was uncertain exactly what he would pursue. Haeckel wanted his trip to be more than a scientific expedition, however. He brought his sketch pads and water colors, and intended also to follow his artistic instincts by spending time in Florence and Rome studying painting before traveling to Naples and Messina for research. In this he was consciously emulating his idol, Johann Wolfgang von Goethe, who also made a famous trip to Italy both to study art and to engage in scientific work. Let me spend a moment on Goethe and two of his disciples, Carl Gustav Carus and Heinrich Bronn, both of whom further developed Goethe’s morphological ideas and who also had a marked influence on Haeckel’s own conceptions of morphology and of the connection between art and science.
When Goethe was in Italy he began formulating ideas that would lead to the development of a new science of his devising, morphology.12 Initially, he postulated an Ur-Pflanze, which would be the archetype of all plants. He believed that with this ideal structure in mind, he would be able to determine the basic form of both those plants that actually existed and those that possibly could exist. Goethe’s ideas came to public fruition in the publication of his Metamorphose der Pflanzen in 1790. In that small treatise, he argued that the various parts of the plant—the stem, leaves, petals, sexual organs, and seeds—could be understood as transformations of one elemental structure, which he symbolically represented as a leaf—or, as he expressed it, “the leaf in its transcendental aspect.” He applied this fundamental conception also to animals. He argued that animal form had two features: an inner kernel and an extrinsic deformation of that kernel. The inner kernel consisted of a topological arrangement of parts, and the deformation resulted from an external accommodation to the surrounding environment. Thus the skeleton of the seal, for example, exhibited a topological pattern of bones shared with land animals, but also displayed particular deformations extrinsically adapting the animal to its aquatic environment. According to Goethe, who was much influenced by Spinoza’s conception of adequate ideas, the archetypal pattern of the vertebrate was an idea actually resident in nature. Moreover, the archetype of the vertebrate, in this scheme, was a force productive of the organism. Goethe understood nature to have the creative powers usually attributed to the deity.
Goethe had originally begun a study of anatomical forms for aesthetic purposes, in order to render the human body in artistic productions. His conception of morphology retained this aesthetic feature. That is, Goethe conceived archetypes or Urbilde as both productive of natural organisms and as necessary for the artist to render nature in the most beautiful fashion. The great artist, in attempting to reproduce natural beauty in his or her painting or poetry would have to understand the very ideas that nature herself employed in a comparable poetic production. Or as Goethe’s disciple the idealist philosopher Friedrich Schelling put it: “the objective world is only the original, though unconscious, poetry of mind.”13 In complementary fashion, Goethe maintained that artistic understanding would reveal, through intuitive perception, the same underlying structures that scientific analysis also strove to display. From the beginning, then, the science of morphology had distinctive aesthetic roots.
Carl Gustav Carus further developed the Goethean doctrine of morphology in his Ur-theilen des Knochen- und Schalengerüstes (1828). In this work, he portrayed in graphic form the archetype of the vertebrate skeleton and its elemental part, the vertebra, which he thought played a role in the plan of vertebrate organization comparable to that played by the leaf in Goethe’s scheme of plant organization (see fig. 4). The primitive vertebra (Urwirbel) could be multiplied and transformed into the backbone, and then into head, ribs, and limbs. Richard Owen in Britain, who read Carus’s work carefully, would elevate the relationships depicted into the concept of homology.14 He would call the repetition of parts within the same animal (e.g., repetition of the vertebrae) “serial homology”; the repetition of the same parts in different but related species, “special homology” (e.g., the digits of the porpoise and bat; see fig. 5); and the repetition of parts in relationship to the archetype or Bauplan, “general homology.” When Darwin interpreted these homological relations as products of descent, he was only deepening the scheme of development cultivated by the likes of Goethe and Carus.
Throughout a long scientific life Carus sought to establish morphology as a science in a strict sense: there had to be laws of transformation of form such that by comprehending these laws one could rationally understand the developmental structure of life. The formation of mathematical laws had long since brought the physical universe to rational order; and German morphologists of the nineteenth century attempted something comparable in the life sciences—though with a naturphilosophisch twist. Carus held that comparative analysis of animal skeletons demonstrated that the elemental figure out of which they could all be geometrically derived was the hollow sphere (Hohlkugel).15 By duplication and deformation the sphere could become a double sphere and then a cylinder, and with the repetition of these forms we could rationally understand the structure of the skeletons of radiate, articulate, moluscate, and vertebrate animals. So, for instance, the elemental vertebra itself can be decomposed into a central sphere and a series of smaller spheres radiating from its periphery (fig. 6). The vertebra of a temporally existing animal, of course, would display the impact of empirical circumstances, though would yet generally conform to its rational archetype. Richard Owen simply followed Carus in his own ideal conception of the archetypal vertebra (fig. 7). This fundamental kind of mathematical idealization of archetypal structures would become a part of Haeckel’s intellectual repertoire; but he would adapt them to Darwinian ends, as I will indicate in a moment. The staggering climax of this tradition of mathematical analysis came in 1942 with the thousand-page edition of D’Arcy Thompson’s On Growth and Form, which likewise examined the structures of biological organisms and their geometrical transformations according to principles of deformation.16 These same kinds of analysis are yet carried on today, though without the presumption that they unlock all the secrets of form in nature.17
In 1854, the Academie des science in Paris announced a prize for an essay that gave the most convincing answers to questions concerning organic development. The winner of this competition was Heinrich Georg Bronn, the man who would first translate Darwin’s Origin of Species into German. In his monograph Untersuchungen über die Entwickelungs-Gesetze der organischen Welt, Bronn argued that fossil deposits recorded the progressive replacement of earlier groups of organisms with later groups better adapted to local environments.18 Species successively replaced one another at various periods in the earth’s history gradually, though not genealogically, as if one species might give rise to another. Extinctions and replacements, according to Bronn, occurred under the aegis of natural forces, which themselves reflected the plan of the Creator.19 In August of 1859, a few months prior to the publication of the Origin of Species, an English translation appeared of the last chapter of Bronn’s monograph. The essay bore the title of “On the Laws of Evolution of the Organic World during the Formation of the Crust of the Earth.” One can see why Bronn’s work piqued Darwin’s interest and evoked Haeckel’s admiration.20
Bronn’s monograph not only conceptually prepared Haeckel to be receptive to the Darwinian proposal, it also offered help in graphically interpreting the new theory. Bronn had included in his prize monograph a phylogenetic tree of the kind Haeckel would later make famous. This illustration, apparently the very first of its kind, depicted the appearance of organisms and their morphological relationships (fig. 8). The large boughs A through G represent the invertebrates, fish, reptiles, birds, mammals, and man respectively. The lower case letters from a to m indicat species at different levels of development and time of appearance in the geological record. Bronn especially wanted to show by this illustration that a main bough, which sprouted earlier in earth’s history, might carry species that appeared later than some on a temporally subsequent bough. In constructing his own phylogenetic trees in the 1860s, Haeckel would have Bronn’s model as an inspiration—though not the only one. (fig. 9)
Bronn provided not only conceptual preparation for the favorable reading of Darwin’s Origin, he present a challenge as well. In his German translation of the Origin, Bronn added an appendix in which he assessed the theory that he had just delivered to the German public. He found Darwin’s theory ingenious, but yet thought it had only the status of a hypothesis. Darwin had shown genealogical transformation to be possible, but he had not shown it to be real. Bronn wrote:
We have therefore neither a positive demonstration of descent nor—from the fact that [after hundreds of generations] a variety can no longer be connected with its ancestral form [Stamm-Form]—do we have a negative demonstration that this species did not arise from that one. What might be the possibility of unlimited change is now and for a long time will remain an undemonstrated, and indeed, an uncontradicted hypothesis.21
In the first flush of enthusiasm for Darwin’s theory, Haeckel would deploy his own habilitation research in southern Italy to render Darwin’s theory more than a mere possibility.