Russian Pioneers in Evolutionary Embryology:
A. O. Kovalevskii and I. I. Mechnikov

In the Origin of Species Darwin wrote that "various pans in the same individual, which are exactly alike during an early embryonic pe-

riod, become widely different and serve widely different purposes in the adult state." "So again," he concluded, "it has been shown that generally the embryos of the most distinct species belonging to the same class are closely similar, but become, when fully developed, widely dissimilar." To add weight to his statement, he cited von Baer's professed difficulty in telling apart the embryos of a long series of vertebrates during the early stage of development.^[90] Although Darwin did not make extensive use of embryological evidence in favor of evolution, he fully sensed its vital importance for the future development of transformist biology. The Origin of Species provided a powerful stimulus for reinvigorated and reformulated embryological research. In Russia the new embryology attracted a group of most promising young scientists, led by Aleksandr Onufrievich Kovalevskii and Il'ia Il'ich Mechnikov. These scientists wanted to answer one of the key evolutionary questions: Does von Baer's description of the vertebrate embryos apply also to the invertebrates? Does embryological evidence support the idea of a fundamental morphological similarity between vertebrates and invertebrates?

A. O. Kovalevskii helped to end the period of exclusive concern with the diffusion and popularization of Darwin's ideas and to open the period of original scientific research in the vast domain of evolutionary thought.^[91] A founder of evolutionary comparative embryology, he never doubted the fundamental correctness of Darwin's theory. Darwin approached embryology as a fountain of scientific information confirming the evolutionary point of view; Kovalevskii treated the evolutionary point of view as an interpretive and integrative principle of embryological research. Kovalevskii entered the annals of science as a thorough empiricist, who wrote careful and remarkably precise summaries of his personal research ventures without showing much inclination toward high-level abstractions and complex schemes of logical constructions. He made it abundantly clear, however, that his preoccupation with empirical minutiae did not lead him to lose sight of the challenging world opened by the evolutionary idea of the morphological unity of animal types: he dealt extensively and minutely with homologies and parallellisms in the embryonic growth of animals belonging to different taxonomic groups. Without stating it explicitly, he made a concentrated effort to build the empirical base for a general explanation of embryonic development—to erase the prevalent pre-Darwinian notion of a morphological chasm separating the vertebrates from the invertebrates.

I. I. Mechnikov made a revealing comparison between Kovalevskii and Ernst Haeckel, as opposite types of evolutionary biologists. Preoccupied with a search for universal laws of biogenetic consequence,

Haeckel showed a clear tendency to ignore products of empirical research which did not fit his grand theoretical schemes and to draw conclusions not warranted by available empirical data. He was not essentially a research scholar but an imaginative synthesizer of current ideas and an architect of grand hypotheses—basing much of his theory of gastraea on empirical data Kovalevskii had supplied. Kovalevskii, by contrast, believed that in more delicate areas of evolutionary research—such as embryology—extreme care should be exercised not to overlook a single empirical detail.^[92] The empirical basis of his embryological research, however, was sufficiently broad to allow him to draw conclusions of larger theoretical magnitude.^[93] By explaining the homologous features of invertebrate and vertebrate embryos he gave evolutionary embryology both a general theoretical orientation and a solid empirical interest.

Kovalevskii's first study—which brought him a magister's degree from St. Petersburg University in 1865—analyzed the growth of the lancelet (Amphioxus lanceolatus), a translucent marine animal. Impressed with the morphological simplicity of the lancelet, which at that time was classified as a vertebrate, Kovalevskii considered the possibility that this marine organism might represent a species occupying a transitional position between vertebrates and invertebrates. His hunch brought rich rewards. Painstaking inquiry showed that the embryonic development of the lancelet falls into two clearly distinguished phases. The initial phase follows the pattern of growth common to invertebrates: it is dominated by an even and nearly complete cleavage of the egg and by the emergence of the blastula, a hollow ball filled with a fluid and bounded by a single layer of cells. The second phase produces the embryo, consisting of an external and an internal germ layer, fully corresponding to the primary layers von Baer had described as a vertebrate characteristic. Kovalevskii's research showed that the lancelet should be classified as an invertebrate of the highest order, and that the embryonic growth of vertebrates and invertebrates is basically similar.^[94]

Kovalevskii then undertook to study the ascidians, immobile creatures fastened to the sea bottom, at that time classified by many as mollusks and by some as worms. By their external appearance, the ascidians do not show even a remote similarity to vertebrates. A closer study, however, provided Kovalevskii with stunning surprises. It showed that the development of the larvae of this organism displayed features characteristic for lancelets and lower vertebrates.^[95] In a later paper, Kovalevskii noted that in their embryonic growth the ascidians were closer to

the vertebrates than any other invertebrate group.^[96] His discovery made a great impression on his contemporaries, more so in the West than in Russia. In The Descent of Man Darwin took serious note of Kovalevskii's interpretation of the embryonic development of ascidians. He stated:

M. Kovalevskii has lately observed that the larvae of the Ascidians are related to the Vertebrata in their manner of development, in their relative position of the nervous system and in possessing a structure closely like the chorda dorsalis of vertebrate animals; and in this he has been since confirmed by Prof. Kupffer. M. Kovalevskii writes to me from Naples, that he has now carried these observations yet further; and should his results be well established, the whole will form a discovery of the greatest value. Thus, if we may rely on embryology, ever the safest guide in classification, it seems that we have at last gained a clue to the source whence the Vertebrata were derived. We should then be justified in believing that at an extremely remote period a group of animals existed, resembling in many respects the larvae of our present Ascidians, which diverged into two great branches—the one retrograding in development and producing the present class of Ascidians, the other rising to the crown and summit of the animal kingdom by giving birth to the Vertebrata.^[97]

A few years after Darwin commented on Kovalevskii's ascidian study, Haeckel incorporated the new discovery into popular literature on the evolutionary idea. He wrote in The Evolution of Man:

Toward the end of the year 1866, among the treatises of the St. Petersburg Academy, two works appeared by the Russian zoologist Kovalevskii, who . . . had occupied himself in studying the individual evolution of some of the lower animals. A fortunate accident had led Kovalevskii to study almost simultaneously the individual evolution of the lowest vertebrate, the Amphioxus, and that of an invertebrate, the direct relationship of which to the Amphioxus had not been even guessed, namely the ascidian. Greatly to the surprise of Darwin himself, and of all zoologists interested in that important subject, there appeared from the very commencement of their individual development, the greatest identity in the structure of the bodies of those two wholly different animals—between the lowest vertebrate, the Amphioxus, on the one hand, and that misshapen lump adhering to the bottom of the sea, the sea-squirt, or ascidian, on the other hand. . . . There can be no longer any doubt, especially since Kupffer and several other zoologists have confirmed and continued these investigations, that of all classes of invertebrates. . . , the ascidians are most nearly allied to the vertebrates. We cannot say the vertebrates are descended from the ascidians, but we may safely assert that . . . the ascidians are the nearest blood-relations to the primeval parent-form of vertebrates.^[98]

Kovalevskii returned several times to the study of lancelets and ascidians, but he also expanded his research interests to cover many additional species of invertebrates, represented mainly by the marine microfauna. In his subsequent studies he discovered the existence of mesoderm in annelids and insects, which provided an additional proof for the basic similarity in the embryonic growth of vertebrates and invertebrates. All these studies added essential information in support of embryonic homologies of vertebrates and widely represented invertebrates. The evolutionary basis of his theoretical orientation, the precision and remarkable skill of his research techniques, and the general significance of his findings assured Kovalevskii of a notable place in the mainstream of biological thought during the early Darwinian era. He not only helped strengthen the hold of Darwin's theory on modern scientific thought but also made a noted contribution to the accelerated growth of modern biology in Russia.

In a paper delivered at the Eleventh Congress of Russian Naturalists, held in St. Petersburg in 1903, V. V. Zalenskii, a distinguished member of the St. Petersburg Academy of Sciences, noted that the evolutionary theory provided "the main stimulus for the entire range of Kovalevskii's research." He noted: "Transcending the limits of pure theory, his research manifested a deep and clear awareness of the great importance of the study of animal evolution, the surest path to solving the basic questions of life that have preoccupied the human mind from time immemorial. An evolutionist by general orientation, Kovalevskii contributed more to the theory of the transformation of organic forms than any one of his contemporaries."^[99] In 1890 the St. Petersburg Academy of Sciences honored Kovalevskii by electing him to full membership. A particularly important event in the history of the Academy, this election marked a major expression of faith in the rich promise of Darwin's theoretical legacy.

Kovalevskii received wide recognition for extending von Baer's theory of the development of vertebrate embryos to all animals: he demonstrated the presence of germ layers among both vertebrates and invertebrates. He showed that during the early phases of their embryonic development all multicellular animals have common features and that strong differences between various types of animals begin to appear only during the later phases of embryonic growth. This, however, was only part of his contribution to embryology. He not only replaced Cuvier's (and von Baer's) rigid division of the animal kingdom into four

types, each with its own separate and fully isolated biological identity, by a unitary system, but also broadened the empirical base of embryology with numerous new concrete facts. He offered, for example, a new empirical method in the study of the early phases in the development of germ layers.^[100]

The by-products of Kovalevskii's research were equally impressive: he discovered the presence of several marine species in the Red Sea, previously thought to exist only in the Mediterranean Sea. In a few cases, his information led to a reclassification of individual species. He added a veritable treasure of empirical facts and generalizations to the scientific knowledge on the formation of the body cavity, digestive canal, nervous system, and vascular network—the central problems of comparative embryology.^[101]

Despite rough beginnings, Kovalevskii's scientific career was a warm story of success and recognition. He was elected an honorary member of almost all Russian learned societies and universities and of a long series of foreign scientific institutions, including the Paris Academy of Sciences and the Royal Society of London. He received two prizes from the Paris Academy of Sciences. The Russian scientific community recognized the full significance of his scientific contributions and honored him profusely. In addition to the universal grandeur of his scholarly achievement, he made a special contribution to Russian science: by relying on a broad evolutionary framework and microscopic methods, he played a major role in transforming zoology from a narrow description of faunistic facts to a theoretically elevated science, dominated by experimental anatomy and experimental embryology. From a descriptive discipline of local interest, Russian zoology became a theoretical discipline with universal appeal. Thanks to a great extent to Kovalevskii's work, Russian zoology achieved impressive results in two activities: the study of phylogeny (based on the evolutionary principle), and the use of modern instruments of inductive research, surpassing the limits of simple observation. Comparative embryology and comparative anatomy became the scientific mainstay of Russian Darwinism.

Kovalevskii was not the only Russian scientist who helped lay the foundations for evolutionary embryology; the contributions of I. I. Mechnikov belonged to the same category of distinguished achievement.^[102] Mechnikov's name is usually associated with the phagocyte theory, built upon a study of intracellular digestion among invertebrates, which helped to explain the origin of multicellular animals and to lay

the foundation for evolutionary pathology. His work in the latter field earned him a Nobel prize, which he shared with the german pathologist Paul Ehrlich.

Mechnikov had much in common with Kovalevskii. Both were models of pure dedication to science, enormous intellectual resourcefulness, and vast reservoirs of energy. But there were also strong differences. Not without some exaggeration, S. Zalkind has pinpointed the differences in their temperaments and styles of work:

Although Mechnikov and Kovalevskii were personal friends and possessed common scientific interests, it would have been difficult to find two men more unlike in mental make-up, character, and methods of research. Mechnikov was a theorerician attracted to general scientific problems, given to making broad philosophical generalizations, impetuous and quick in his conclusions, sometimes apt to disregard facts but self-confident and persistent in the attainment of his aims. Kovalevskii, on the other hand, was an empiricist, tackling only concrete tasks, avoiding (we may even say, fearing) "all that lofty theoretical stuff," but at the same time extremely precise and thorough in his observations. A modest, mild and yielding man in everyday life, he was firm and indomitable in scientific disputes concerning facts which he knew well and had verified many times.^[103]

In 1863, as an eighteen-year-old student at Kharkov University, Mechnikov wrote an essay on the Origin of Species in which he scrutinized the pivotal ideas of Darwin's thesis, particularly the derivation of the struggle for existence from Malthus's law of mathematically formulated discrepancy in the growth rates of population and food resources. He also thought that the present existence of many lower organisms disproved Darwin's theory. If Darwin were right, he said, "these beings, the initial steps in the organization of life, would have begun to change a long time ago, giving place to more advanced forms." When pushed against the wall, Mechnikov argued, Darwin did not hesitate to rely on spontaneous generation to account for the emergence of species that could not be accounted for by evolutionary processes. All this, however, did not prevent him from concluding that the theory presented in the Origin was destined to have a great future and from considering himself one of its most ardent supporters. Mechnikov submitted his manuscript to F. M. Dostoevsky's journal Time, but the journal went out of existence before it could act on the new acquisition. The manuscript waited until 1950 to be published in a volume of Mechnikov's essays.^[104]

After intensive study under several leading German biologists and a passing interest in the embryology of insects, Mechnikov made the

Mediterranean marine invertebrates his main research concern. But, even in this activity, he resisted a dose adherence to the Darwinian theory: he clung steadfastly to his original idea that Darwin had advanced too many general ideas of a purely hypothetical nature. To Kovalevskii, Darwin's theory served as the incontestable basis of comparative embryology; to Mechnikov, it belonged to the realm of challenging hypotheses requiring careful experimental testing. Mechnikov did not hesitate to criticize the evolutionary conclusions Kovalevskii had reached in his studies of lancelets and ascidians as devoid of a solid empirical basis. He was particularly critical of Kovalevskii's claim to have established the unity of the embryonic growth of invertebrates and vertebrates. Nor did he approve of Kovalevskii's assertion that among lancelets the process of invagination leads to the formation of the digestive tract. He contributed articles to scholarly and popular journals in an obvious effort to discredit Kovalevskii's theses. His conclusions, however, showed that he had left the door open for a more conciliatory attitude toward the idea of the embryonic unity of the animal kingdom.^[105] The deeper he became immersed in embryological research, the more closely he became identified with the theoretical foundations of Darwin's legacy. During the 1870s his own empirical research led him to a full acceptance of Kovalevskii's position.^[106]

From 1865 to 1869 Mechnikov wrote about thirty papers on the embryonic growth of an unusually large number of animal species, mainly invertebrates. He helped confirm the discovery that all animals have two basic germ layers—ectoderm and endoderm—thus giving added strength to the idea of the evolutionary unity of the animal kingdom.^[107] His research concentrated on comparative embryological studies of animals whose morphological affinity had not yet been established. Some of his conclusions were not upheld by subsequent research; yet his evidence in favor of the general relationship of the Echinodermata, the Enteropneusta, and the Chordata had gone unchallenged.

At the beginning of his scholarly life, Mechnikov was not inclined to tie comparative embryology to Darwin's evolutionary theory. As late as 1869 he wrote: "The comparative history of [embryonic] development deals with facts from which it draws direct conclusions without considering the origin of various species."^[108] Soon after this pronouncement, Mechnikov, influenced by both Kovalevskii's research and his own, became an evolutionary embryologist in the full meaning of the term. Despite his persistent criticism of certain aspects of Darwin's theory, it would not be an exaggeration to say that after the early 1870s Mech-

nikov's entire scientific work and all his theories were part of a brilliant search for the deeper meanings of the scientific legacy of the English naturalist.^[109]

As he accepted Darwinism as a broad theoretical orientation, Mechnikov saw the basic source of its power in the combination of a historical view of nature, a comparative approach to biological phenomena, and an identification of purposiveness in the organic world with the processes of adaptation as a means of survival. If his work had weaknesses, they stemmed, not from a lack of scholarly dedication and experimental skill, but from the unique features of his temperament: unsettled and excitable, he moved too swiftly—particularly in his embryological work—from one research undertaking to another to do justice to all of them; his embryological research, for example, covered representatives of almost all major groups of invertebrates and some vertebrates.^[110]

In 1867 the St. Petersburg Academy of Sciences awarded the first Karl von Baer Prize for outstanding work in biology. A special committee selected Mechnikov and Kovalevskii to be the first recipients of the coveted prize. The committee noted that the work of each scholar showed distinct excellence: Mechnikov was honored for having produced a "complete and integrated" study, Kovalevskii for the "diversity of subjects" covered.^[111] The committee noted with approval Kovalevskii's discovery of embryonic links between vertebrates and invertebrates.^[112] Leon Bliakher, a modern historian of nineteenth-century embryology, has argued that Mechnikov must also be counted among the discoverers of these links, soon to be recognized as the foundation of evolutionary embryology.^[113] Karl von Baer, the leading anti-Darwinian scholar of his age, served as the ranking member of the selection committee. Perhaps because Mechnikov showed signs of reluctance to link embryology with the Darwinian theory of evolution, von Baer praised his scholarship more than Kovalevskii's. Kovalevskii's full and consistent identification with Darwinism was well established from the very beginning.

The scientific work of Kovalevskii and Mechnikov represented the crowning point in the reception and early application of Darwin's ideas by Russian natural scientists. But how were these scientists regarded by the Russian scientific community? Although the St. Petersburg Academy of Sciences published their papers, they had difficulty in finding suitable employment. At the time when St. Petersburg and Moscow provided the most coveted academic positions, Kovalevskii and Mechnikov had no choice but to seek teaching positions in provincial universities,

equipped with poor laboratories and libraries and wanting in intellectual stimulus for sustained scientific work. Mechnikov's candidacy for a position in the Medical and Surgical Academy in St. Petersburg proved futile. He served sixteen years on the faculty of the newly founded Odessa University, resigning in 1882 to avoid the grueling pressure of academic intrigue and student unrest. In 1887, before he had reached the peak of his scientific career, he left Russia to join the Pasteur Institute in Paris, where he remained until his death in 1916. A. O. Kovalevskii spent twenty-two years of his academic career in Kazan, Kiev, and Odessa universities fighting the depressing monotony of provincial isolation by extensive correspondence with western European embryologists, frequent scientific trips to the Mediterranean Sea, and cooperative research ventures with eminent foreign scientists.

Whatever the reason for the negative results of their initial search for academic employment in St. Petersburg and Moscow, the two young scholars were primarily responsible for the preeminent role of provincial universities in making Russia one of the early centers of empirical studies in organic evolution. It is most likely that the professional hardships of Kovalevskii and Mechnikov did not result solely—and perhaps not even primarily—from their identification with Darwin's theory. It should be remembered that the academic market—and the growth of employment opportunities—was controlled by the limited purse of the Ministry of Public Education much more than by the efforts of the scientific community to keep up with new developments in individual disciplines.

Among the new breed of evolutionary embryologists N. D. Nozhin occupied a unique position: he began as a searching scientist of great ambition and talent and ended as an ideologist dedicated primarily to emancipating his country from both the decaying feudal law and the burgeoning capitalist relations. After having studied chemistry under Robert Bunsen at Heidelberg University, he moved to Tubingen University, where he studied zoology under H. G. Bronn, the German translator of the Origin of Species, but not a Darwinist. In 1863 he went to Italy to conduct research on the embryonic growth of selected species of Mediterranean fauna for the purpose of answering the question of a possible morphological link between vertebrates and invertebrates. It was in Italy that he established close relations with A. O. Kovalevskii^[114] In his spare time he translated Fritz Müiller's Für Darwin into Russian; this work was generally acclaimed as the first successful effort to combine meticulous embryological research with Darwinian transformism.

The study, according to Mechnikov, marked the first scientific effort to base embryological research on Darwin's theoretical principles.^[115] The St. Petersburg Academy published Nozhin's lone scientific paper, a study of coelenterates (primarily medusae).

During his sojourn in Germany, Nozhin participated in several circles of Russian students eager to find ways of bringing modern political and social ideas to their native land. At this time Nozhin began to think of Darwin's theory as a source of ideas for a unitary picture of the evolution of the universe, both natural and social. He viewed Darwin's work as the culminating point of nineteenth-century science, and science as the only source of sound guideposts for purposive action in social development. Like the champions of nihilism, he preached "a visionary faith in science" as the true power of reason and declared that "all scientific knowledge in the hands of its honest servants stands in direct opposition to the existing order" and that "in the world there is only one evil—ignorance—and only one way to salvation—science."^[116] He argued that only by knowing the laws of nature could man widen the humanistic base of social existence. However, he rejected the struggle for existence as the moving force of evolution; he called it an aberration, a pathological force exercising a negative influence on both natural and social evolution. He called Darwin a "bourgeois-naturalist" for his emphasis on competition—rather than on cooperation—as the mainspring of biological and social development.^[117]

Nozhin supplied populist sociology—to which N. K. Mikhailovskii gave a fully crystallized form—with guiding ideas and logical structure. While Kovalevskii saw in Darwin's theory a fruitful method of scientific analysis, Nozhin saw in it the culminating point in the evolution of the modern scientific world view—a triumphant victory of reason over metaphysical mysticism and religious dogma. Nozhin must be counted among the first Russian scientists to express two thoughts that found strong followers during the subsequent decades: first, not the theory of the struggle for existence but the elimination of supernatural causality in the development of nature was Darwin's major contribution to science; and second, sociology owed a great debt to Darwin's theory—not to the notion of natural selection, but to the unitary developmental scheme and rational models for social analysis.

Among the Russian scientists who supported Darwin's ideas during the 1860s Sergei Usov, professor at Moscow University from 1868 to 1886, occupied a unique position. His translation of Friedrich Rolle's

Darwin's Theory of the Origin of Species, published in 1865, had the unique distinction of having been the first book in the Russian language to offer a comprehensive presentation of Darwin's theory. The author provided a detailed but simple explanation of Darwin's basic principles and made an effort to link the theory of organic evolution with the idea of progress. Usov's doctoral dissertation—Taxonomic Units and Groups (1867)—devoted a special chapter to the history of evolutionary theories in biology.^[118] The chapter gave a systematic and detailed analysis—the first in the Russian language—of Lamarck's evolutionary theory. In Russia, as in the West, the triumph of Darwin's theory opened the gates for the rediscovery of Lamarck. Usov scrutinized both the similarities and the differences between Lamarck and Darwin. The aim of his analysis was to provide a historical legitimation of Darwin's theory: to show that the new evolutionary idea was built upon the solid foundation of accumulated biological knowledge. Usov played a major role in making Moscow University a true bastion of Russian Darwinism. Although he regarded Darwin's theory as a triumph of modern biological thought, he did not hesitate to point out that some of its grand theoretical conclusions needed stronger empirical support.