August Weismann: Development, Heredity, and Evolution [Pilot project, eBook available to selected US libraries only] 9780674286832

Citation preview

August Weismann

August Weismann Development, Heredity, and Evolution Frederick B. Churchill

H a rva rd U n i ver si t y Press Cambridge, Massachusetts London, England 2015

Copyright © 2015 by the President and Fellows of Harvard College All rights reserved Printed in the United States of America First printing Frontispiece: Th is portrait of Weismann was painted by Otto Scholderer, a German painter who practiced in London. Weismann knew both Otto and his brother Emil in Frankfurt. Writing to Emil in 1907, Weismann commented that Otto visited him at his Freiburg home on a daily basis while painting the portrait and that it “had from the beginning excited the admiration of his family” (“Das Bild, das er damals von mir malte hat die Bewunderung meiner Familie eregt”). Weismann to Emil Scholderer, 16 June 1907. Klaus Sander, ed., August Weismann (1834–1914) und die theoretische Biologie des 19. Jahrhunderts. Urkunden, Berichte und Analysen. Freiburger Universitätsblätter, vols. 87, 88. Freiburg: Rombach Verlag, 1985. Library of Congress Cataloging-in-Publication Data Churchill, F. B. (Frederick B.) August Weismann : development, heredity, and evolution / Frederick B. Churchill. pages cm Includes bibliographical references and index. ISBN 978-0-674-73689-4 (alk. paper) 1. Weismann, August, 1834–1914. 2. Biologists— Germany— Biography. I. Title. QH31.W45C48 2015 570.92— dc23 [B] 2014035141

To Sandra Riddle Churchill

Contents

Preface

ix

I Explorations into Natural History and Development 1. Growing Up and Early Research 2. The Age of Development

3

15

3. Becoming an Embryologist: Insect Development 4. Studies in Descent, Part I: 1863–1876

29

50

5. Studies in Descent, Part II: 1873–1876

91

6. Daphnia Research and the Ecology of Lakes

123

7. From Germ Layers to the Germ-Plasm: The Study of 147 Hydromedusae, 1877–1883 8. From Egg to Heredity: 1876–1885 9. A Perspective on Heredity

179

201

10. Carl Nägeli and Inter- and Intragenerational 224 Continuity 11. The Emergence of Nuclear Cytology

238

II An Architectonic View of Heredity 12. A New Perspective on Heredity: Continuity 277 and Heredity 13. Transmission of Adaptations and Evolution: 1885–1890

311

Contents

viii

14. Polar Body Research: 1887–1891 15. Protozoa and Amphimixis

345

361

16. A Model for Heredity: Das Keimplasma, 1892 17. Controversies and Adjustments: 1893–1896

387 431

18. The Germ-Plasm and the Diversity of Living Phenomena: 470 1890–1900 19. Seasonal Dimorphism in Butterflies and Parthenogenesis 489 of Drones 20. Adapting the Germ-Plasm: 1900–1914 21. A Mechanical Model for Evolution Epilogue

583

Acknowledgments Index

547

569

Appendix: Works by August Weismann Notes

506

681

685

Color plates follow page 386.

575

Preface

Few contemporary historians and philosophers of science know of August Weismann. Still fewer life scientists know of his achievements in biology, although they may have heard his name in connection with the scientific turmoil over the processes of heredity and evolution in the period between the publication of The Origin of Species (1859) and the beginning of World War I (1914). This was a time when the scientific world was arguing with more opinion than exactitude over the operation and mechanisms of biological evolution. To promote Darwin’s theory of natural selection or to assemble some combination of competing causes, be they physical, chemical, or organically more complex, were issues foremost in the minds of both zoologists and botanists. What follows is an intellectual biography of Weismann, which places him on the same level of importance in shaping the biology of evolution as historians have placed other frequently heralded luminaries in the world of biology. Overall, the text of this biography is a chronological narrative that follows Weismann’s life from birth to death. It is based to a large extent on Weismann’s publications, correspondence, Tagesbücher, and other relevant university documents now housed in the Freiburg University Library. The analysis focuses on the development of the entire corpus of Weismann’s work as manifested in his publications. It necessarily also examines the relevant publications of contemporary biologists with whom Weismann interacted. In general, this means that each chapter introduces the setting for important nonscientific and institutional events in Weismann’s life during the period covered by that chapter and then moves on to the relevant biological developments during that same period. Occasionally the two are interwoven within the same chapter.

x

Preface

Included in the narrative are vignettes of Weismann’s scientific contemporaries who interacted with his science during the period being covered. This is particularly the case with the careers of his exact contemporary Ernst Haeckel and his antagonists Rudolf Virchow, Herbert Spencer, Oscar Hertwig, George John Romanes, Theodor Eimer, and Richard Semon. The parallel and diverging careers of Weismann and Haeckel are so important (and are documented in already published correspondence) that they are treated in an early single section, which then makes it possible to refer to Haeckel throughout Weismann’s life. There are points when August Gruber, Weismann’s brother-in-law and long-term institutional colleague, becomes an important part of the history of Freiburg University. This chronological pattern, however, must be interrupted at key points when the surrounding study of life develops rapidly and initiates a change in the whole of biology as well as in Weismann’s own career. Thus, Chapter  2 pictures the state of biology at the time Weismann enters the science; likewise, Chapters 9 through 11 compare in a general way the nature of heredity and the rise of nuclear cytology. This latter development, which demarcates a transitional period in Weismann’s research career, terminates Part I and initiates Part II of the biography. The publication of Weismann’s best-known work, Das Keimplasma, also begins a period when Weismann is criticized from many directions and responds in kind. The important cytological research of Theodor Boveri and the closely connected advent of Mendelian genetics are biological developments that challenged Weismann’s germ-plasm theory, which the by then older Weismann had to accommodate. Important to Weismann, and to most prominent German zoologists of the day, was the university institute. These housed both museums of natural history with material important for zoological research and scientific laboratories, providing the benches for the training of doctoral students and the space and equipment for the development of the early scientific careers of a small number of teaching and research assistants. Of course, it also served as the forum in which Weismann lectured to university students, many of whom were medically inclined, pursued his technical research, and spent most of his academic career. Unlike Haeckel, Weismann stuck to his last at his institute; and unlike much of Haeckel’s writings, most of Weismann’s published work addressed technical matters rather than popular problems about the role of the evolution of mankind. However, like Haeckel, most of Weismann’s major publications were quickly translated and published in English, an indication of how important his work and ideas were for English and American biologists.

Preface

I have written this text with historians and philosophers of science particularly in mind. It focuses primarily on Weismann’s ongoing research at his zoological institute in Freiburg, Germany. I also hope that biologists with an interest in an important period in the discipline’s past will find an intellectual biography of this important nineteenth-century zoologist both interesting and valuable, particularly for the period between the publication of Darwin’s The Origin of Species and World War I. There is to my knowledge no secondary source in English, German, or French that covers Weismann’s scientific career in such a comprehensive fashion. The few surveys of biology that cover this period generally consider only Weismann’s germ-plasm theory and then only in a cursory fashion. Yet his career and accomplishments were well known during his lifetime, and he focused attention on how the fields of embryology, heredity, and evolution must ultimately be understood as a comprehensive whole. Ernst Gaupp, a young contemporary of Weismann in Freiburg, provided a digest of his scientific work in his August Weismann: Sein Leben und Sein Werk (1917). Although he was cognizant of Weismann’s uncompleted “Vita Propria,” Gaupp’s study was more a summary of Weismann’s mature germ-plasm theory than a historical account of his career. The historical writings of Peter Bowler, the sweeping and detailed history of biology by Ernst Mayr, and the informative conference on the sesquicentennial of Weismann’s birth edited by Klaus Sander all offer important but constrained historical reflections of certain aspects of Weismann’s career but do not provide a historical analysis of its development. Much the same can be said of the chapters in Leonid I. Blacher’s The Problem of the Inheritance of Acquired Characters (1982),1 Reinhard Mocek’s Die Werdende Form: Eine Geschichte der Kausalen Morphologie (1998), Lynn K. Nyhart’s Biology Takes Form (1995), and Robert K. Richards’s The Tragic Sense of Life: Ernst Haeckel and the Struggle over Evolutionary Thought (2008). Although the last three comment on Weismann within the context of their own valuable analyses of the period, they do not explore the historical development of Weismann’s science. Nevertheless, Weismann looms as the most important of the German contributors to a synthesis of microscopic and evolutionary sciences at the end of the nineteenth century. It is my intention that this biography should fill this lacuna. Only with a fuller picture of Weismann’s career and accomplishments can we understand the development of zoology both in Germany and in England between 1859 and 1914. As this biography proceeds, a chronological map will unfold of the different personages and materials as they reacted with Weismann’s separate

xi

xii

Preface

accomplishments. One pattern that becomes clear is that Weismann tended to work from one biological problem to another. Often these problems were immediately related to one another, as when he moved from the study of hydromedusae (i.e., hydroids) to heredity theory; at other times, they were less directly connected, as when he moved from dipteran development to butterfly biogeography. Whatever he pursued was meticulously presented, and all his studies fed into his understanding of heredity, development, and evolution— and ultimately into his germ-plasm theory. Personal circumstances and accidental opportunities may have shaped his choice of study, but Weismann appeared always to have kept the bigger picture in mind. The same may be said of his choice of dissertation topics for his advanced zoology and occasional anatomy students, fifty-nine in all.2 But in these respects, his research and institutional careers may not have differed much from later twentieth-century patterns.

August Weismann

I E x plor ations into Natur a l History a nd Dev elopment

1 Growing Up and Early Research

August Weismann was born in 1834 into what is commonly called the “Bildungsburgertum,” or educated middle class, in the free city of Frankfurt on the Main, some thirty kilometers up river from where the Main flows into the Rhine. The Main valley in itself was highly agricultural, and heavily cultivated according to contemporary accounts. South across the Main and connected by a single bridge sat the much smaller town of Sachsenhausen, and visible to the north were the low-lying Taunus Mountains. At the time, Frankfurt, one of four free cities of the German realm, consisted of roughly 55,000 people. The city contained narrow, twisting streets and many partially timbered medieval buildings.1 Its medieval walls with the exception of some towers had been torn down during the occupation of Napoleonic troops, and the city leaders later took advantage of the vacated space to turn it into green promenades. It was a quaint city run by its own diet, more commercial than industrial, and proud of its past. Frankfurt was also the birthplace of Johann Wolfgang Goethe, the most noteworthy of many illustrious citizens, who had died two years before Weismann’s birth. When Weismann was twelve, the city became the site of the Frankfurt National Assembly, which strove to find a representative solution for the disparate and competing states in the German realm. Although the city did not possess a traditional university, it sported the locally established and funded Städelsches Kunstinstitute, or art institute, with its large collection of paintings and drawings, and the similarly local Senckenberg Naturforschende Gesellschaft, or science society, which focused on physiology and anatomy. Both institutions were important to Weismann during his upbringing.

4

Explorati ons i nto N atur a l H is to ry a nd D eve lo p me nt A mid-twentieth-century genealogist of the large Weismann family traced August Weismann’s lineage, with some uncertainty, back to Valentin Weissmann [sic], a citizen (“bürgerlicher Einwohner”) who lived in Weierburg in Nieder Östereich and who apparently was martyred for his Lutheran (“evangelisch”) beliefs. According to the genealogy, his undeterred followers remained overwhelmingly Lutheran. A large number of the men appeared to serve as teachers in their careers. The Frankfurt stem of the family, however, began with a banker, Immanuel Friedrich Weismann. His first son, Jean (Johann) Konrad August Weismann (1804–1880)—who would be the father of the zoologist—became in 1831 a teacher and later a professor at the city’s gymnasium. Jean married Elenore Elizabeth (Eliza) (1803–1850), daughter of Christian Leopold Lübbren, the mayor of and civic official in the northern city of Stade situated close to the mouth of the Elbe River, and of Marie Margarete (née Römhild). Jean and Elenore conceived four children: Leopold Friedrich August (1834–1914), Agathe Marie Julie (1836–1900), Julius Heinrich (1838–1863), and Therese Marie (1844–1857). August Weismann wrote of all three of his siblings, and he also produced two short autobiographical sketches, which were published for specific purposes. He also began but did not complete an autobiography, which he entitled Vita Propria: Erinnerungen aus altester Zeit.2 It is to this latter work I turn to recount his boyhood years and early career up to the summer of 1871. At this point he, unfortunately for historians, ended his account. He terminated this more orthodox autobiography in 1913, roughly a year before his own death on November 5, 1914. When exactly he began or ended the text is not clear; needless to say, the reader must take care in interpreting this life story written at the age of seventy-nine, but the names and facts contained therein do appear to be accurate. We learn of an idyllic childhood life as painted by the seventy-nine-year-old Weismann. The family first lived near the Eschenheimer Turm, the tall fourspired tower that had been spared during the Napoleonic occupation of Frankfurt. Gardens existed between the neighboring houses. The promenade fashioned in place of the city wall was easily accessible for play and walks, and the former moat provided a brook for additional play. The family soon moved to Mainzerstrasse, which followed the remains of the old fortifications. It was a new street, lined by handsome new houses, a standard setting for the German educated middle class of the period. Weismann remembers, from that period, the bugle sounds of the post, music played on the sides of the streets, a paver resetting street stones, his rocking horse in a sunny room, his father playing the piano (“perhaps some Bach”), and cherry trees behind their house. These

Growing Up and Early Research

Figure 1.1. Weismann and his sister Agatha in 1839. August Weismann (1834–1914) und die theoretiche Biologie des 19. Jahrhunderts. Urkunden, Berichte und Analysen. Freiburger Universitätsblätter, vols. 87, 88. Freiburg: Rombach Verlag, 1985. Helmut Risler, “August Weismanns Leben und Wirken nach Dokumenten aus seinem Nachlass,” p. 24.

cheerful scenes allowed him to recall his siblings, Agatha and Julius, and walks with his parents in the neighboring fields before the city that were full of flowers and butterflies, which fascinated him. Later, Weismann recalls moving to a different house near the Gallustor by the former fortifications on the west side of the city. It lay in the neighborhood

5

6

Explorati ons i nto N atur a l H is to ry a nd D eve lo p me nt of the large three-story house that served as the Prussian consulate. Still as a young boy, Weismann recalls hiding at the bottom of the steps of the consulate and watching King Frederick Wilhelm IV of Prussia, who was visiting Frankfurt, climb them. The king appeared to the inconspicuous youngster to be a slow moving, heavy, and unwell man. (Frederick Wilhelm IV was born in 1795 and ascended to the Prussian throne in 1840; so he may have been in his late forties at the time. Later he was declared insane and superseded by his brother, who became regent.) On a cultural level, it is evident that his mother had a lasting influence on the young Weismann in many ways. She was a trained artist in both aquarelle and oil, and even before her marriage she was painting formal portraits. Apparently she had learned to copy old masters in the style of the Düsseldorf Academy, which had begun to emphasize realism and nationalism.3 In turn, she encouraged her son to draw, and by the time he reached fourteen years of age, Weismann took art lessons given by a Jacob Becker at the local Städelsches Kunstinstitute. His mother also encouraged him to play the piano at an early age, which led him to take lessons from a Herr Heymann. He readily took to music. He recalls listening to Haydn’s Creation and to Beethoven’s “Pastoral” symphony, but at the age of 13 he opined that the cuckoo call in the Andante of the latter was unrealistic. By the age of 16 he claims to have understood all of Beethoven’s symphonies except the Ninth. It is interesting that he barely mentions his siblings receiving the same attention, but later he mentions that Julius had a nice singing voice and should have been encouraged to develop it. It happened that this music teacher was also a butterfly collector and interested his young pupil to develop his own collection and to learn not only the names of the specimens he caught but also about their larvae. When the family moved outside the city to Taunusstrasse, Weismann had at his disposal the outlying fields of clover and other flowers and more butterflies. He mentioned that his father was not entirely happy about his involvement with nature, considering it only a hobby for rich people, but that his mother continued to support his interest and his grandparents (presumably on the maternal side) gifted him Bertuch’s book of natural history.4 When the teachers at the higher levels of his school did not support his collecting of insects either, he turned to plants and put together a herbarium. Only after an older friend, a Dr. Bagge, at the time a candidate in theology and a good botanist, began taking him on collecting trips, did Weismann deeply pursue the subject. Together, with a druggist friend, they went on collecting trips to the southwest toward Mainz

Growing Up and Early Research

and southeast toward Aschaffenburg. Weismann felt the range of locations provided him with a great number of species, and he began to learn their scientific names, but regretfully, he later added, not their physiology. Weismann also enjoyed traveling with the entire family when all the children were old enough to make such adventures by a combination of boat, carriage, train, and foot. He recounts traveling by boat, then by foot, up the Ahrtal, a trip to Königstein in the Taunus, and exploring the Odenwald from Miltenberg over Amorbach to the Rhine and back to Frankfurt. His later descriptions are not long or detailed, but what is noteworthy is that he remembers best the living nature about him. He recalls the names of butterflies, rock crystals, and flowers, to a point when his mother remarked at one point that he appeared to miss an unusually beautiful sunset for the sake of a meadow in bloom.5 Spring of 1848 came early in March. Having just entered his fifteenth year, Weismann was tuned into some of the political events sweeping Germany and the rest of Europe. When the “Vorparlament” met in Frankfurt, Weismann was aware that Friedrich Hecker, a political radical from Baden, stayed in the house next to his. He remembered Hecker speaking to a sympathetic crowd from the window. Adding to the same recollections, Weismann mentioned that he also heard “many” speeches in Paulskirke, the site that served as the seat of the Frankfurt National Assembly. He claimed that at the time he was an outright “Republikaner” who wore a “black-red-gold freedom cockade” and a “blood red scarf ” and with his friends played at war in the hedges of the neighborhood. He added, interjecting his later convictions, that he gradually realized the “dark side” of the republic when he visited the barricades before the fighting really began. “It seemed to me that the arbitrariness of the foolish multitude was too uncertain a leader,” and “I soon became a convinced supporter of Prussian authority.” With these thoughts he finished his paragraph by asserting that “if we no longer want to remain a joke with other people and if we also want to be of value to the world, then we must become united into a solid whole with a functioning government on the top of which stands one man and that can be no other than the Prussian king.” 6 During the same spring of 1848 Weismann visited his maternal grandparents in Stade near the mouth of the Elbe. Whether this was an attempt on his parents’ part to get him out of the turmoil of Frankfurt or not is unclear from Weismann’s sixty-four-year retrospective. It was a long trip alone for a fifteen-year-old. He went by steamboat from Mainz to Köln, then by train to Hannover. On the ride he had struck up a friendship with the wife of a noncommissioned officer, and when they reached Hannover, she offered him the

7

8

Explorati ons i nto N atur a l H is to ry a nd D eve lo p me nt extra bed in her room at the inn where she was staying. He felt the situation was awkward ( genierlich), but he assured his potential reader that nothing unusual transpired. He slept soundly, and they parted in a friendly way the next day. Weismann then went by train to Hamburg and by post to Stade. Travelling through sand dunes and heath and passing many small, colorful houses, he finally arrived at his destination situated on a winding stream, which led into the Elbe. The new setting and the large delta area of the Elbe clearly impressed him. Although there were a few steamships most of the shipping on the river still consisted of large sailing ships bound upstream to Hamburg or returning to the ocean. “It was a magnificent view to see such a ship, covered with full sails from bottom to top traveling up the Elbe; the swans gliding calmly and easily behind and often following one another.”7 He describes a trip with his “dear and friendly grandmother” and his aunt Auguste by boat to Hamburg where he was again impressed by the ships in the harbor presenting a “veritable forest of masts.” Unlike those in London or Marseille, he commented, the ships in Hamburg were not forced to lie together in a confined space. “All the masts, yardarms, ropes, etc., present a confusion of lines, and reach so high in the heaven, that that alone really impressed us.”8 When Weismann returned to Frankfurt he wrote a theme for school with the title “a day in Hamburg.” His father, who was his German professor at the time, read the theme aloud praising it to his class. On account of the death of his grandfather, Christian Leopold Lübbren, Weismann again visited Stade, this time with his father and younger brother Julius. He relates an adventure that he and Julius had in a boat with the plan to return to Stade by way of the winding stream leading from Stade to the Elbe. The tide in the Elbe was such that they could not complete their return and had to overnight in the half-open cabin of a small fishing vessel. Because the boat’s occupants could only speak “plattdeutsch,” it was hard for them to communicate. Nevertheless the ending of the story was a happy one. When they returned the next morning their father was waiting for them on the shore, but he did not say a word. Weismann surmised that he was so relieved that he could not reprimand them. Weismann felt it important to indicate the living standard of both his grandfathers. His father’s father, Immanuel Gottlieb Friedrich Weismann, moved to Frankfurt where he worked in a bank. He lost much of his estate when, during the Napoleonic wars, soldiers were quartered in his house. As a consequence the family, consisting of two successive wives, who bore him collectively nine children, had to go into debt in order to succeed in raising five of them

Growing Up and Early Research

to maturity.9 Of the four surviving boys, Weismann’s father, the oldest, and the next two became teachers, the fourth broke ranks and became a banker. Three of the four, including the banker, remained in Frankfurt; the fourth taught in Coburg. His mother’s father, as mentioned above, was evidently well off and became the mayor and local official in Stade. He was able to leave Weismann’s mother a modest sum upon his death, which in turn helped Weismann in his training as a zoologist when she passed away at the early age of forty-seven. Weismann strongly felt the loss of his mother when, shortly after he had turned sixteen, she died. She has been “a true friend” not only to him but to Agatha and Julius, and she could hardly be replaced by his father, who tended to be strict and constantly scolding. As Weismann mentioned a number of times, his mother had opened up the world of music and fine arts to him and encouraged him in his collection of butterflies and plants. As the eldest child, Weismann felt that he received the brunt of his father’s rebukes. Later he learned to appreciate his father more, and when Weismann lived elsewhere, he visited him frequently in Frankfurt. Both his mother and younger sister Theresa died of typhus, the former in December 1850, the latter in 1857. Typhus had been a disease of long standing in Europe, documented as far back as the sixteenth century. Dr. G. Spiess, a medical friend close to the family and father to one of Weismann’s closest friends at school and university, was called on to treat both mother and sister. He followed the older practice of first giving her “normal salt, which irritated the intestines even more—at any rate did not help, and then came the hunger treatment with oat gruel.” His mother craved food while she wasted away from starvation; in retrospect, Weismann was outraged. By the time seven years later his sister Theresa contracted the disease, Weismann was a young physician and had become aware of more modern treatments, such as allowing the patient to drink milk as both a soothing and nutritional medicament. Alas, he was in Rostock when she came down with the disease, and by the time he returned to Frankfurt, Theresa was nearly gone. Fifty years later, Weismann still felt strong regret that he could do nothing to save her. Soon thereafter, his other sister, Agatha, and his father also came down with typhus. However, this time Weismann could insist on supplementing Dr. Spiess’s gruel with milk. Both patients survived, and Weismann wrote a bitter retrospective passage about how the new treatment was already known by the clinically more advanced physicians of the time. Before turning to Weismann’s scientific career, it makes sense to extend briefly this genealogical account rather than trying to place individuals in

9

10

Explorati ons i nto N atur a l H is to ry a n d D eve lo pme nt chronological sequence later on. While striving with the legal system— unsuccessfully as it turned out—to win his inheritance from his mother which was held by his aunt Dorotea Römhild on his mother’s side, Weismann was introduced to the Gruber family in Genua. While his elaborate negotiations over what turned out to be a nonexistent last will and testament were coming to naught, the Gruber family, a large German family led by a successful merchant, Adolf Gruber and his wife Julie Schönleber, provided a substitute home for him in Genua. Among them he partook of family life and frequently played the piano for everyone; in fact, he became most attached to the eldest daughter, Marie (known as Mary) (1848–1886), who often vocally accompanied him. The two married in 1867. Over the course of twenty-one years, the couple lived in Freiburg, where Weismann habilitated and rose to be a full professor of zoology with his own institute. Together, they raised six children: Therese (b. 1868), Hedwig (b. 1870), Elise (b. 1871), Bertha (b. 1873), Meta (b. 1876), and Julius (b. 1879). Meta died early, but the rest of the children lived to raise families of their own. Julius became a professional pianist, composer, and conductor of the Freiburg symphony orchestra. The others we frequently hear of in Weismann’s published letters.10 His wife, Mary, died early at the family retreat in Lindenhof on the Bodensee.

Gymnasium and Göttingen When he reached the primary level at the gymnasium, Weismann read Sophocles and Horace (presumably in the original Greek), and took German with his father. Later he claimed to have found all these studies stimulating. He struggled a bit with mathematics but turned down the offer by his mother to receive special assistance. In retrospect, he found that he “lost nothing thereby.” Later after becoming a young doctor, he made an effort to study a book by the French mathematician Leonhard Euler on the algebraic solution to geometrical problems, but he neglected to report how his independent study went.11 By the time he was thirteen years old, Weismann was taking piano lessons with a Franz Messer,12 and attending lectures on chemistry and physics with Rudolf Christian Boettger, a highly regarded teacher at the Physicalishen Verein in Frankfurt and later the director of the Senckenberg Naturforschende Gesellschaft.13 Boettger’s lectures in chemistry stimulated Weismann enough to consider professionally entering the field. His father asked for advice about this plan from Friedrich Wöhler, who had been born near Frankfurt and after his appointment in chemistry at Göttingen was an occasional visitor to his

Growing Up and Early Research

home city. Wöhler advised the young student that before becoming a chemist he should secure his living by first becoming a physician. The advice turned out to be prophetic, and the contact with Wöhler may have been one of the reasons that in the fall of 1852 Weismann attended Göttingen to pursue his medical doctorate. Weismann took a course with Wöhler after matriculating, and the contact, although he did not say so, may well have shaped his enduring mechanistic belief about life. Limited finances were also a reason for his not pursuing botany or zoology, even if the latter had been independently available. Nevertheless, Weismann looked forward to studying the acceptable medical sciences, particularly those of anatomy and physiology. By the mid-nineteenth century, as the historian H-H. Eulner has pointed out, “histology, comparative anatomy and embryology were included in physiology and therefore part of the responsibility of the anatomists.”14 By the mid-nineteenth century the trend in medical anatomy was working toward comparative anatomy, which in itself might form a bridge to zoology. In addition to the presence of Wöhler, Jakob Henle, a former prosector for Johannes Müller and a renowned microscopical anatomist in his own right, had just been appointed professor of anatomy at the university. New opportunities would open up for the beginning medical student Weismann. Throughout his four-year stay in Göttingen, encouraged by Alexander Spiess, childhood friend and the son of his family’s doctor, Weismann roomed with the highly reputable professor of surgery Wilhelm Baum and his family. Wilhelm Baum was a professor of surgery at Göttingen between 1849 and 1875. At the time, Baum also lectured on ophthalmology.15 Weismann mentions that Baum was a mild, thoroughly good man who was artistic, musical, and devoted to his students. Weismann remembered Frau Baum as an unusual woman, who had good intuitions about people, who also was highly musical and played well four handed on the piano. They had two daughters (ages eighteen and sixteen) and a son. So for four years Weismann felt at home and gradually, through the family, met many local professors.16 As for his friends, he mentioned Spiess, who was attending Göttingen at the same time, and Theodor Otto von Heusinger from Marburg, who later returned to his native city as a physician. As anticipated, Weismann heard the anatomy lectures of Henle, but he considered them dry and too confined to the refined details of human structure. Despite the fact that Henle had transformed his lectures into the more exciting examination of comparative anatomy when he taught at Heidelberg, he resisted without explanation such an

11

12

Explorati ons i nto N atur a l H is to ry a nd D eve lo p me nt expansion of the subject in Göttingen. Weismann, of course, heard Wöhler’s lectures in chemistry, but after having taken Boettger’s chemistry course in Frankfurt, he felt he learned nothing new at the university. In addition, Weismann discovered that the renowned Göttingen physiologist Rudolf Wagner gave engaging lectures but was ill, so left much uncovered; the famous Handbuch by Johannes Müller did not provide a substitute, as Weismann considered it by the early 1850s to be out of date, although it still contained many valuable ideas.17 In retrospect Weismann remembered the four years as a medical student more for the music he participated in than the sciences he pursued. Despite his disappointing lectures, Henle provided Weismann and his friends with opportunities to play quartets, the piano, and even sing at his home. Weismann mentioned that he particularly enjoyed playing Chopin’s Nocturnes and Scherzos but not the “technically unattainable Études.” Weismann also alluded to a four-year attachment he had with a “fine, clever girl.” Both realized, however, that their relationship could not continue, for Weismann had to leave Göttingen and could not seriously consider marriage before he found his way in the world.18 Weismann’s inaugural dissertation at Göttingen concerned the formation of hippuric acid in humans.19 An expanded version of his work as it applied to herbivores received a royal award from the medical faculty.

Rostock and Chemistry After passing his doctor’s examination with a First in July 1857, Weismann accepted a position as an assistant with Benjamin Theodor Thierfelder, who had just become professor of medicine at the city hospital in Rostock. His fee for ser vice provided him with 100 Thaler and living expenses. In part because Weismann was not really interested in extending his clinical experiences and in part because the work often seemed foolish (närrisch), he allowed himself to be enticed by his “best friend” to join Professor Franz Schulze’s chemical laboratory. Here, he was set up with a small laboratory room, which also served as his living quarters, where, with Schulze’s help, he carried out a chemical examination of the fluctuation of salt in the seawater of the Ostsee.20 This work won a prize from the faculty. Again in retrospect, he was most impressed by the professor of comparative and pathological anatomy Hermann Friedrich Stannius, author of an important text on vertebrate anatomy. The professor was friendly to Weismann and other advanced students.

Growing Up and Early Research

As soon as the semester was completed, Weismann traveled with a friend from Frankfurt through the East Sea to Copenhagen. Fifty years later, he would still wax with romantic enthusiasm for the blue sea, the medieval fortifications of the royal city, and the starry night sky as the two traveled by boat and carriage through Flensburg and the straits between Schleswig and Holstein. At the time, both of these duchies belonged to Denmark. Weismann claimed to have learned very little from his chemical research about either urea or seawater. In retrospect, he felt that he did not have the “patience and exactness of a chemist for experimenting.”21 Nevertheless, we can reasonably assume that after spending four years in Göttingen studying and writing a dissertation in chemistry and a year pursuing a chemical problem in Rostock, he had learned to think like a chemist. He read the classic chemical works by Fourcroy, Vauquelin, Wöhler, and Liebig, and he learned to juggle the atomic quantities of carbon, hydrogen, nitrogen, and oxygen in the production of urea and its products. He determined the specific weight of seawater with changes in temperature, wind, and barometric measurement and with an eye to the changes of salt concentration in different locations and under different physical conditions. His minor publications in both areas received encomiums from the respective faculties of two universities. Never again would he dabble in chemistry, but philosophically his chemical experience formed the bedrock to all of his later biology. This generalization includes his research on seasonal polymorphism and his examination of the changing distributions of variations over time as well as his cytological research, which eventually led to his theories of heredity.

Return to Frankfurt Weismann left Rostock and traveled home to Frankfurt when his younger sister, Therese, came down with typhus. As mentioned earlier, his mother and sister had succumbed to typhus, and then Agatha and his father also had come down with the disease. Weismann, now on the scene, insisted on a milder regimen that included a milk diet. It was a treatment he had learned from Thierfelder in Rostock. What had been a lethal disease for his mother and younger sister became a serious but treatable disease for the rest of the family. At the same time that further research in Rostock left his thoughts, Weismann toyed with the possibility, encouraged by his father, of earning a living as a physician in Frankfurt. During the summer of 1858, he went through the motions of studying medicine under some of the great stars on the faculty

13

14

Explorati ons i nto Natur a l H is to ry a nd D eve lo p me nt of medicine in Vienna. He visited their lectures but interacted on a personal level only with Carl Ludwig, who at the time was professor of anatomy and physiology in the Austro-Hungarian capital, and with the chief physician at the general hospital whose daughter, Weismann mentioned, later became an opera singer in both Karlsruh and Munich. Most memorable of his summer experiences was a lengthy overland mountain excursion, which started in the region of the Grossglockner, descended to the Dolomites, and ended after an unspecified transit in the Austrian Ötztal. Here he aspired to continue, but on account of the weather he was unable to climb Mount Similaun, which at 3,602 meters is the highest peak on this stretch of the Italian-Austrian border. The trip included acquiring rugged mountain boots, frequent glacier climbing, and many hours of ascents and descents with a personal guide, who at one point saved him from sliding out of control down a glacier. Weismann mentions one comical moment: when he was dozing on the mountainside, he was awakened by the peering, bearded face of a large mountain goat. The trip ended in a more relaxing way when he turned to the visual arts in the Munich galleries and museum. The heritage from his parents of many Rhineland travels and the tutelage from his mother in both music and art weighed more strongly on Weismann’s seventy-nine-year memory than the scientific experiences gained at this time.22

2 The Age of Development

By the time Weismann was sixteen in 1850, some of the most important nineteenth-century scientific approaches to the understanding of life had already been initiated. Above all, there emerged a single-minded focus on embryological development throughout the animal kingdom, which was associated with the defining accomplishments of Karl Ernst Baer in the third and fourth decades of the century. These were paralleled in exquisite detail by Baer’s contemporaries Christian Pander, Heinrich Rathke, and Johannes Müller. Continuing full swing after the publication of Darwin’s The Origin of Species in 1859, descriptive embryologists, not yet steeped in experiments, assumed a dominant role in driving zoological and botanical research to the end of the century. Their successes invited the next two generations of students who aspired to become professionally trained anatomists and zoologists to follow their lead. Derived from descriptive embryology and born of the same desire to understand the microscopic details of the organism, the cell emerged as the basic unit of organic structure and function. Articulating the cell theory in twin publications, Matthias Schleiden and Theodor Schwann provided a visible and variable unit out of which descriptive and later experimental embryologists fashioned their own tapestries of whole organisms. Contemporaries such as Müller, Jacob Henle, and younger investigators such as Albert Kölliker, Robert Remak, and Rudolf Virchow seized upon the cell as the unit for understanding such diverse organic phenomena as development, reproduction, and disease as well as the contrasts between sexual and asexual generation. Little was envisioned about organic form without reference to the cell. At the same time, while investigating life throughout the world, many naturalists began studying in depth the alternating forms and processes of different generations of the

16

Explorati ons i nto Natur a l H is to ry a nd D eve lo p me nt same species. The meaning of this apparent violation of the classical formulation of sexual reproduction was not immediately obvious. Investigations on this score by Japetus Steenstrup, Richard Owen, Karl von Siebold, Rudolph Leuckart, and Thomas Henry Huxley did much to reveal the overall complexities of animal reproduction. The lion’s share of work in all three of these areas of investigation was done after the Napoleonic period in Germany at the reformed state universities. Essential to all three—descriptive, cellular, and reproductive biology—was the continued development of the science of microscopy, about which we will have much to say in later chapters.

The Study of Development: Baer It is historically curious that three medical students on the Baltic periphery of German cities initiated the earliest descriptive studies of development in the nineteenth century. Pander, Baer, and Rathke knew each other as students and followed in one another’s footsteps as each advanced in his early career. They were in turn assisted by the Dorpat, later Königsberg, anatomist and physiologist Karl Burdach and by the Würzburg anatomist and physiologist Ignaz Döllinger.1 The combination of both anatomy and physiology in the same Lehrstuhl may have simply spoken to the time and limitations of the medical faculties at small universities in the German states, but it also may have encouraged thinking in terms of embryology. Both Burdach and Döllinger wrote about development and considered it a necessary dimension to their belief in the unity of science and life itself. Their influence on Baer, Pander, and Rathke should not be underestimated. To capture the impact of descriptive embryology on zoology at the end of the fifth decade of the century, when Weismann began his own embryological studies, one cannot do better than focus on the achievements of Karl Ernst von Baer. Baer is remembered for two outstanding accomplishments in embryology. The first entailed his successful demonstration of the mammalian egg in 1827, which had previously eluded many scientists; once realized, it unified the general picture of reproduction among all higher organisms.2 The second encompassed a two-part work entitled Über Entwickelungsgeschichte der Thiere: Beobachtung und Refl exion that traditionally shaped the rise of modern descriptive embryology.3 A careful examination of this substantial work of nearly 600 pages and seven plates of multiple figures (four of them in contrasting red, yellow, and black) helps explain why the book was justly elevated to the status of a revered exemplar during the second half of the century.

The Age of Development

As a totem, the entire work possessed an awkward structure about which to navigate. Its organization and its interrupted publication resulted in two quite differently constructed parts. The first, appearing in 1828, was in turn separated by two distinct objectives. The second was cobbled together by the publisher in 1837 from a series of Baer’s lectures describing additional observations on chick development and an extended detour by Baer on the development of reptiles, numerous mammals and their different eggs, and finally fish. An introductory letter credits his friend Pander with the initial inspiration for his embryological studies. Its casual construction notwithstanding, the totality of the work evinced both an observant and a thoughtful investigator who made great strides in the factual details of the development of not only the chick but also mammals including humans and other vertebrates. Although he focused on the chick, Baer’s observations reflected the importance of comparative embryology. Equally significant was the fact that the second half of part one consisted of general principles of development that could easily be adjusted over time as the factual details changed with new investigations. As Jane Oppenheimer pointed out many years ago, the secondary title to Baer’s Entwickelungsgeschichte stressed that the work contained both “Beobachtung und Reflexion”— observation and reflection.4 Turning first to the Beobachtung or the observation of details of animal development, we find that Baer focused initially on the twenty-one days of development of the “Hühnchen” or chick. He segmented this process into three unequal sections. It is clear that Baer must have used a microscope as well as the lens that had led to his discovery of the mammalian egg. This instrument is inferred from his fine analysis of the early germ formations: the stratification of germ layers, the primitive streak seen from various angles, the bending and pinching off of the embryo from the rest of the egg, and the formation of the neural tube and primitive gut as well as, by the end of the first day of observation, some of the somites. Baer continued identifying tissue layers, the aorta, the notochord, the cephalic region, the foregut, and the vessels stretching throughout the egg, and he noted the torsion of the body onto its left side to accommodate the future expansion of the heart and to allow for greater flexion in the cephalic region. “The turning of the embryo onto its left side,” Baer rationalized, “is a very important moment in the formation of the fetus, for on it depend many changes, especially the metamorphosis of the heart in the interior.” As a consequence, he explained, the yolk sack and umbilical vesicle (Nabelblast) also lay on the left side. The sequel of this asymmetry Baer also found in the lizards, snakes, all birds, and all mammals he

17

18

Explorati ons i nto Natur a l H is to ry a nd D eve lo p me nt examined, and he documented his personal observations by indicating that only two of more than a hundred chicks that he had studied had turned instead onto their right side.5 Thus, his studies continued through the detailed examination of development and were embroidered with many explanations and comparisons with other species through the twenty-one days of chick development and 140 pages of compelling prose. Baer’s account of the twenty-one days of chick development was brilliantly descriptive and suggestively comparative. At that time, however, zoology was struggling with the relationship between the development of individual particulars and the general outline of taxonomy of the whole. It was essential that Baer instruct his readers on how to deal with both the particulars and generalities of zoology. It is within this context that Baer presented his readers with a lengthy discussion of the principles of embryology. It was the distilling of these principles into Scholia and Corollaries that made his Entwickelungsgeschichte the premier statement of the goals of embryology for the rest of the century. In resorting to six Scholia and associated Corollaries, Baer embraced a device used by Euclid and Newton, but his lawlike summary for embryologists appeared much more a lesson from his empirical observations than from deduced generalizations. What follows is an abbreviated account of these Scholia and Corollaries.6 Scholium I, “On the Certainty of Observation of the Embryo,” dealt with Baer’s recognition that young embryos are coarser in outline than older ones and they simply do not exist in miniature below the resolving power of the common microscope of the day. Scholium II, “The Formation of the Individual in Relation to Its Surroundings,” relates Baer’s observation that during development the embryo increasingly separates itself from the rest of the egg and then sequentially incorporates into itself the remains of the blastoderm, other egg structures, and some of the extraembryonic membranes. The same self-promotion also applied to the production of the ovum and offspring so that “generation is here a direct relocation [Verlängerung] of growth past the boundaries of the individual[,] and propagation is nothing more than an extension of growth beyond oneself.”7 Here, Baer employed a metaphor about reproduction and heredity that would reemerge in zoological texts for the next fifty years. Scholium III, “Internal Development of the Individual,” is where Baer points out that development is a process of differentiation from the homogeneous to the heterogeneous. This pattern in itself was not surprising, but Baer recognized its simple empirical onset by describing the initial formative

The Age of Development

process as consisting of the appearance of two plates: the animal and vegetative. He further observed that each plate then divided to produce four germ layers: the skin, the muscle, the vessel, and the mucous layers. Baer found that these structures emerge through the transformation rather than new formations. “Each organ is thus a modified part of a more general organ, and in this respect one can say that each organ is already contained in the fundamental organs.”8 Scholium IV, a much longer and more complex section, presents the generalized “Scheme That Vertebrate Development Follows.” As the germ layers become distinct, Baer noted that they grow into a double tube, each with its internal and external cylinder. These tubes are positioned above and below the longitudinal axis indicated by the notochord. Scholium V, the longest of the scholia, addresses the taxonomic “Relationships of the Forms Which the Individual Assumes in the Different Stages of Development.” This was clearly the climax of the Scholia, and this and Scholium VI were the only ones translated into English by T. H. Huxley.9 Baer first set out to refute the widely held law of parallelism, which derived its name from the postulated parallel between the hierarchical scale of taxonomy and the teleological progression of development. He argued not against a limited correlation between the two, but denied the existence of a universal, uniserial hierarchy of all organic forms that could be traced to embryogenesis. Instead, Baer generalized the relationship between the type and grade of development in terms of four different laws of development. (1) The more general characters of a group of organisms appear earlier in development than the more specialized characters. (2) The less general forms develop from the more general forms. (3) Every embryo of a given animal form rather than passing through other forms becomes separated from them. (4) The embryo of a higher form never resembles the adult of any other form, only its embryo. Scholium VI concluded the first volume of the Entwickelungsgeschichte by generalizing Baer’s results: “The History of Development of the Individual.” Baer asserted in italic type that this “is the history of its increasing individuality in all respects.”10 Only within the context of the preceding five Scholia and the body of empirical studies presented in the first 140 pages could this aphoristic generalization exert its full impact. Finally, Baer closed his text with a reflective passage that reminded the reader that cosmic design—that is, a teleological perspective—was still very much a part of the study of life. As deftly translated by Huxley twenty-five years later, this key passage concluded:

19

20

Explorati ons i nto N atur a l H is to ry a n d D eve lo pme nt there is one fundamental thought which runs through all forms and grades of animal development, and regulates all their peculiar relations. It is the same thought which collected the masses scattered through space into spheres, and united them into systems of suns; it is that which called forth into living forms the dust weathered from the surface of the metallic planet. But this thought is nothing less than Life itself, and the words and syllables in which it is expressed are the multitudinous forms of the Living.11 Many readers have noted how the concluding paragraph in Darwin’s The Origin of Species is reminiscent of—though not metaphysically equivalent to—this passage from Baer. Baer’s fame did not rest exclusively on the first part of the Entwickelungsgeschichte, but there is no need to survey the second part beyond that which has already been discussed. Baer’s interest in tracing the development of the fertilized egg and the separate organ systems of the chick became the model, providing the organizational themes for later texts on descriptive embryology. From a more general perspective, Baer’s extraordinary efforts to juxtapose the development of birds, reptiles, mammals—including humans—and some invertebrates established the importance of comparative embryology for anatomy, taxonomy, and eventually for the study of phylogenies. Above all, Baer’s understanding of early development in terms of the dynamics of germ layer development provided his contemporaries and later generations with both the empirical grounds for establishing homologies and a powerful tool for focusing on the mechanical processes of development. Rathke, followed by Johannes Müller and Robert Remak, Carl Bergmann, Rudolf Leuckart, Wilhelm His, and many others, carried the descriptive tradition well into the 1860s and beyond. A recent biographer of Baer, Boris Raikov, has delved into the wealth of manuscripts left in St. Petersburg where Baer lived out his long life after moving from Könnigsberg in 1834.12 From the archives, Raikov has determined that when Baer took up the position of prosector then professor of anatomy at the Prussian university in Könnigsberg, he also took up the task of rebuilding its zoological museum. Early in his academic career, Baer gave popular lectures based on his teaching human anatomy with an eye to comparisons with other vertebrates and his maintaining the museum with its growing collection of specimens. He lectured on chick development twice in 1821 and on reproduction in 1822. Shortly thereafter, he published the first volume of a proposed

The Age of Development

but never completed collection of more formal lectures on physical anthropology, which comprised a standard introductory course on the description of adult human anatomical systems and organs.13 Baer never published the proposed second volume, but a manuscript of its contents shows that Baer intended to discuss embryology along with anatomy and physiology. Above all, the manuscript reveals how immersed at the time Baer was in the Naturphilosophie of Friedrich Schelling and Lorenz Oken, which urged an understanding of the transcendental laws of nature through the developmental processes. Later, Baer would repudiate the implied philosophical system, but he pursued the empirical details of animal development—in part because he could not accept the recent mechanical and preformationist conclusions about development of the Swiss and French chemists Jean Louis Prevost and Jean Baptist Dumas.14 Of immediate value to Baer’s exploration of the chick was the fact that he drew upon Pander’s earlier dissertation on the development of the same organism. Both Pander and Baer depended upon a discovery by Döllinger about how to open the chick egg to reveal the living embryo inside. In need of an exemplary of invertebrate development, Baer could peruse Rathke’s recently published examination of the embryology of the crayfish.15 Besides promoting the onset of descriptive embryology, Pander and Rathke, like Baer, pointedly described the earliest development of the embryo in terms of germ layers. Both argued that there were three basic germ layers, although Baer argued for four. Three or four layers notwithstanding, the implied mutual mechanical interaction of germ layers served the same function in the formation of primitive embryonic structures. Pander’s account was brief but described the appearance of these layers: “Actually a unique metamorphosis begins in each of these layers and each hurries toward its goal; although each is not yet independent enough to indicate what it truly is; it still needs the help of its sister travelers, and therefore, although already designated for different ends, all three influence each other collectively until each has reached an appropriate level.”16 Not only did Pander identify interacting germ layers, but his experience and words strongly suggested that each layer was “designated for different ends,” that there was a unique specificity in their actions and in their developmental destiny. This specificity challenged the research of embryologists for the rest of the century. In her detailed account of the rise and fall of the germ layer doctrine, the embryologist and historian Jane Oppenheimer demonstrated how the germ layers became the principal element in a mechanical explanation of

21

22

Explorati ons i nto Natur a l H is to ry a n d D eve lo pme nt development during the third through sixth decades of the century. Thereafter, descriptive generalization was transformed into an inflexible causal biological principle as its apparently valid generality joined forces with the theoretical controversies over evolution theory incited by the publication of The Origin of Species. By the 1870s, experienced embryologists such as T. H. Huxley and Edwin Ray Lankester in England and above all Ernst Haeckel in Jena advocated the strong taxonomic connection between the specific destiny of germ layer formation and evolution.17 Oppenheimer’s conclusion, which only concerns us for the very end of the century, comes with the maturation of experimental embryology during the first third of the twentieth century, which demonstrated the complete falsity in the claim of a lawlike specificity of the germ layers.

The Rise of Cellular Biology At the very end of the century and in an all too brief overview of zoology and its successes during the past hundred years, Weismann, then sixty-five and a renowned and experienced zoologist, commented on the consequences of the improvement of the microscope. “This goes for the discovery, which forms the basis of our examination of the fine structure of living forms for the discovery of the cell by Schleiden and Schwann.” He recounted, “it goes, however, even more for the science, which bases itself on cells, for histology or the doctrine of tissues, that are bound together for the different functions of the higher animal body in particular ways and organized cellular masses. Elsewhere it was also first the microscope, which allowed us to recognize the lower and smallest animal world as single celled organisms.”18 Alas, this comment is brief, part of a frustratingly short summary. Not surprisingly, it reflects the three areas of Weismann’s own interests but distorts the history of cell theory to the point of ignoring the contentious issues over cells between the forties and sixties, in which Weismann in a small way became also involved. In a comprehensive historical survey of cell theory, Henry Harris recovered the complicated narrative about the recognition of and discussions of that “fine structure” of organisms, which preceded and followed the well-known monographs by Schleiden and Schwann at the end of the 1830s. Harris brought together the various strands in a way that is less unidirectional, providing a picture of a research frontier in the throes of debate and flux.19 In retrospect, it may seem a tedious and excessively elaborate story, but Weismann’s early

The Age of Development

and later works were unavoidably involved. Harris provides a historically sophisticated picture of the many microscopists who were examining the fine structure of both botanical and zoological materials. From seventeenth- and eighteenth-century observations by Anton van Leeuwenhoek to Lazzaro Spallanzani on what we now call “microorganisms,” and from the detailed identification of fission in the cells of the plant meristem by Barthélemy Charles Dumortier and Hugo von Mohl, and from the casual illustration of bodies within cells by Robert Hooke and the 1833 paper by Robert Brown, who with a compound microscope identified what he called the “nucleus,” cells became increasingly identified as central to the study of histology. The Czech histologists at the German university in Breslau—Jan Evangelista Purkyné and his students, particularly Gabriel Gustav Valentin—worked in the 1830s with achromatic compound microscopes to describe cells in great detail; Purkyné in par ticu lar generalized the structures to include both animals and plants. Johannes Müller and his students and associates in Berlin were zeroing in on the same structure. What becomes evident from Harris’s valuable account is that when Matthias Schleiden and finally Theodor Schwann generalized these and their own observations in 1838–1839, their “cell theory” reflected a broad research frontier, not an isolated discovery. What made their theory so attractive was that it not only provided descriptions of structures but embraced a developmental conception of growth, differentiation, and pathological abnormalities in cells. Schwann’s monograph in particular captured the contemporary efforts to generalize upon the fundamental building blocks of all life. It also explicitly accepted endogenous, exogenous, and binary fission as different modes of cell reproduction. Harris writes repeatedly of “Schwann’s mistake” of embracing multiple modes of cell reproduction, and it took until the mid1850s, following the work of Franz Unger, Robert Remak, and Rudolf Virchow, to establish that only binary fission of a cell into two daughter cells provided the only pattern of reproduction. As such, however, their conclusion turned out to be too simplistic. The revised cell became doctrine, and cytologists refocused their energies on the importance of the nucleus, the nucleolus, and other cellular structures and processes. By this time, Weismann had matriculated in Göttingen, had listened to Henle’s excessively descriptive lectures in anatomy, “ohne alle leitenden Gedanken” (“without any guiding ideas”),20 and had written his dissertation under the guidance of the physical chemist Friedrich Wöhler in the philosophical faculty.

23

24

Explorati ons i nto Natur a l H is to ry a n d D eve lo pme nt

The Alternation of Generation and Other Modes of Reproduction During this thirty-year period between 1830 and 1860, another issue focused the attention of biologists. It was more a collection of phenomena that resulted in a new perspective than a set of overall guiding principles or a single theory about fine structure. This research revolved around the different modes of reproduction in individual organisms. The Canadian biologist and historian John Farley has written cogently and in detail about the subject in the first half o