Like Goethe, Haeckel cultivated his passion for painting and poetry while roaming through southern Italy. On the islands of Ischia and Capri, he joined the colony of German artists and poets; and he more often reached for his easel and palette before his microscope. (See some of his canvases done in Italy, e.g., figs. 10 and 11). He even thought of giving up his scientific research for the life of the bohemian. What drove him back to work on his habilitation was the thought of his fiancée, Anna Sethe, his first cousin, whom he left back in Berlin (fig. 12). She became ever more vital to his being, and if he were to finally return to her, he had to complete his habilitation research.
Through the early fall of 1859, Haeckel examined the many marine organisms that he dredged up from around Messina, finding new species and even orders of invertebrates never before described. At the end of November, with just a few months left for his research in Italy, he finally decided to focus on just one group of animals, the almost unknown radiolaria—a large class of one-celled marine organisms that secreted unusual skeletons of silica.22 (The skeletons of these animals, it might be noted, make up twenty percent of the muck that lies on the sea floor.) Haeckel’s own mentor in Berlin, Johannes Müller had written a short monograph on these animals. It was his final publication, appearing just after his suicide in 1858.23 Haeckel had the foresight—or perhaps just the simple desire for remembrance—to bring the tract with him to Italy. During the course of his own research, the monograph became his "gospel," and he virtually memorized it.24 But Müller's work, it was clear, had been preliminary and much remained for an ambitious researcher—especially to provide concrete meaning for that ever nebulous claim of systematists that the several groups of organisms they treated were more closely or distantly related. When Haeckel produced his own monograph on the radiolaria—greater in length and breadth of consideration, more beautiful by far than that of his teacher—he dedicated it to Müller, so that natural piety linked Müller's tragic end with Haeckel's glorious beginning.
Haeckel wrote Anna, his fiancée, to describe the creatures that would become his constant companions, though at one-thousandth to eight-hundredths of an inch in diameter they were hardly companionable (e.g., fig. 13):
The radiolaria are almost exclusively pelagic animals, that is, they only live swimming on the surface of the deep sea... Their body consists of a hard and a soft part. The hard part is a siliceous skeleton, the soft is mostly a spherical, small, round capsule surrounded on all sides by an outcrop of many hundreds of exceptionally fine filaments, by which the animals moves and nourishes itself.25
Under his microscope ever new radiolarian species began to appear; so that by the spring he was able to ship back to Berlin specimens of some 101 species never before described.26
Shortly after returning to Berlin, at the end of April 1860, Haeckel arranged to work on his collection at the Berlin Zoological Museum, where he prepared a report to be presented to the Academy of Sciences and then finished his Habilitationschrift.27 These essays carefully described the new species he had discovered and analyzed their internal structure, something never before done and which remains today the starting point for further explorations with the scanning electron microscope. (One might compare Haeckel’s figures with recent micrographs, lest one assume that Haeckel’s imagination had taken over: fig. 14) He determined the radiolarians to have a soft body consisting of a central capsule, with a minute inner vesicle (Binnenblase), and surrounded by smaller vesicles (Bläschen), through which radiate a great number of stiff, thread-like pseudopodia (e.g. Heliosphaera actinota, fig. 15). Depending on the family, the skeleton either surrounds the central capsule (as with the solitary Polycystinae) or extends into the capsule (as with the Acanthometra and the colonial Polycystinae).28
In determining the structure of the radiolaria, Haeckel had to be adept in handling the microscope—the radiolaria, after all, were the size of a pin-head. He had the ability to gaze through his microscope with one eye, while simultaneously using the other to draw the figures. To get their intricate geometry correct, he would stud a potato with small rods, and then stabilize his model with the artist's sense of balance and proportion.29
Neither the readers of the Academy report nor of the Habilitationschrift would have been prepared for the large two-volume monograph Haeckel produced in 1862, Die Radiolarien (Rhizopoda Radiaria). The first two exercises announced a scholar of competence and promise, the latter showed the promise already brilliantly fulfilled. The monograph, which so astonished Darwin and which would be awarded the prestigious gold Cothenius medal of the Leopold-Caroline Academy of German Scientists (1864), displayed many extraordinary features through its over 570 pages of the first volume and the 35 copper plates of the second. I will mention just two of the more significant aspects: the works relation to Darwinian theory and its aesthetic character.
In his great monograph, Haeckel attempted to arrange his species into a "natural system" based on homology.30 The two principal comparative axes for homological arrangement concerned the relation of the skeleton to the central capsule (either completely external to it, or partly inside it) and the forms of the skeleton itself (or its absence). On this basis Haeckel distinguished, as they fell into pattern, some fifteen natural families.
Haeckel said he was inspired to attempt a natural system because of the extraordinary book he had read while preparing his specimens—Über die Entstehung der Arten im Thier- und Pflanzen-Reich durch natürliche Züchtung, oder Erhaltung der vervollkommneten Rassen in Kampfe um=s Dasyn by the English naturalist Charles Darwin. Haeckel first looked into Heinrich Bronn's German translation of Darwin’s Origin of Species while at the Berlin Museum in the summer of 1860, just after he had returned from Messina. Being an anti-authoritarian—in his later days to the point of dogmatism—Haeckel was probably enticed to read the new work because curators at the museum regarded it as a "completely mad book."31 Though anti-authoritarian, Haeckel was not foolish; so it is not surprising that no mention of Darwin appeared in his Academy report in the fall or in his Habilitationschrift. It may be, though, that the full impact of the Origin had not struck home during the composition of those pieces. In November 1861, while laboring full bore on his great monograph, he again opened up the Origin; and as he related to his fiancée Anna, he "buried" himself in it.32 From that fertile ground he emerged newly born for Darwin's theory, and the zeal of his conviction never cooled through the later days.
What kept Haeckel=s enthusiasm for evolutionary theory glowing was the special contribution he thought he could make to establish it empirically. He seems to have been especially piqued in this respect by Bronn’s claim, in the appendix to his translation, that Darwin had not demonstrated the reality of transmutation, only the possibility. Haeckel argued that the radiolaria provided the desired empirical support for the new theory of evolution, since the relatedness of species within families bespoke genealogy and the transitional species joining families confirmed it.33 Yet in developing his argument for the reality of genealogical transformation, Haeckel invoked two conflicting principles that led to different understandings of the natural system that he claimed his research uncovered.
One principle, which allowed him to organize his specimens into families, genera, and species was that of progressive skeletization. The other principle was derived from a Goethean aestheticized morphology, this had to do with principles of symmetry and harmony.
In the ordering of his 35 copper plates for the second volume, Haeckel began with Thalassicollida (fig. 16 ), which had a central capsul but no skeleton; he then moved to Spiculosa, which had no organized skeleton, but only spiculae jutting from the surface of the capsule; and so on through progressive degrees of skeletization. All of this seemed quite within a Darwinian paradigm of evolutionary progress. Yet Haeckel proposed another principle by which to understand the relationships among the morphological types. And this harkened back to the older morphological tradition. He suggested that the Urtypus of the phylum, the one that might have given rise to others, was comparable to Heliosphaera (fig. 15). This became the archetype whence all of the fifteen families might be derived. Immediately after mentioning the new considerations that Darwin introduced into zoology and his own declaration of apostleship, Haeckel wrote:
A continuous red thread passes through the entire series of these forms, so that I am already prepared to make the effort to represent graphically the connections and many-sided relations of all these forms in one genalogical table of relatedness. From this table all other possible forms might be derived. I see such an Ur-radiolarium as a simple spherical lattice from which spiculae radially protrude and in whose internal area the central capsul floats. The lattice is suspended on pseudopodia extending everywhere. We actually find this Ur-Typus in the genus Heliosphaera. As a model, we can take Heliosphaera actinota, with its twenty symmetrical separated spiculae (according to Müller’s law).34 Of course I am far from maintaining that all the radiolaria must be derived directly from this form, only that it can be shown how, as a matter of fact, that all these extensively developed forms can be derived [abgeleitet] from such a common fundamental form.35
The idea that the descent relationships might operate according to various mathematical deformations of the basic sphere was quite in the older Goethean tradition of morphology, comparable to Carus’s derivation of the form of the vertebra from geometrical arrangements of the sphere. Haeckel even suggests, as Goethe had, that once the archetype had been discovered through comparative analysis, the naturalist would be able to derive not only the forms actually existing, but also those that could possibly exist.
In later monographs, Haeckel's illustrations would more closely unite the morphological and the genealogical orders into one evolutionary tableaux of systematic arrangement. In the Challenger volumes (published in the 1880s), Haeckel concluded that the Ur-Typus, the archetype and original organism from which all the radiolaria descended, was a spherical form with radiating pseudo-podia, but without a skeleton.
The archetypal structures that Haeckel detected as the basic forms of different animal groups, the original forms of the progenitor organisms, could be comprehended, as Goethe had earlier suggested, only by the mind’s eye. But such forms could yet be rendered in their essence by the artistic hand. And this is why, for Haeckel and other biologists of the nineteenth century—and even today—artistic sensibility reveals what mechanical productions, like photographs, can only obscure. The dramatic and exotic beauty of Haeckel's illustrations and their artful arrangement would in future play decided roles in persuading his readers of the evolutionary theory that would stand ever more strongly behind them.