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This chapter examines how both first- and second-cohort pragmatists participated in the debates over the causal factors of evolution that accompanied the reception of August Weismann’s work in the 1890s. Weismann, a biologist, argued in the mid-1880s that the hereditary substance was confined to what he called the “germ-plasm,” which was isolated from the rest of the body. One implication was that acquired characteristics could not be inherited, undermining the neo-Lamarckian theories of American scientists such as Edward Drinker Cope and Henry Fairfield Osborn. These discussions were of interest to William James and Charles Sanders Peirce because of their opposition to Herbert Spencer (also a neo-Lamarckian) and Peirce’s 1893 essay “Evolutionary Love” should be interpreted as contributing to the factors of evolution debates. John Dewey also followed these debates, applying key concepts from an 1893–95 dispute between Spencer and Weismann to his early work in ethics and social psychology.

This chapter discusses how cell theory revolutionized the concept of the individual. A plant or an animal was no longer conceived of as a singular entity constructed out of cellular tissue. What was important was that the cell was alive — it possessed all the attributes of life. The cell state, dawn of protistology, the term “cell” as a misnomer, and the theory formulated by the Freiburg zoologist August Weismann are discussed.

This chapter focuses on the ways British socialists interpreted evolution. As Fabianism and Marxism became more prominent, the Lamarkian and anti-Malthusian politics of men like William Morris and Peter Kropotkin became increasingly marginalized. H.G. Wells, a one-time admirer of Morris and sometime Fabian, took particular exception to Morris's anti-Malthusian assumptions. This became all the more so in light of his conviction that the German cell biologist August Weismann had undermined Lamarckian inheritance. Without the inheritance of acquired characters any hope of a significant human evolution on a relevant time scale was lost. Further, Wells became convinced that Weismann's theory of panmixia further undermined Morris's vision of socialism. Any ‘epoch of rest’ that ameliorated selective pressure would cause biological degeneration. George Bernard Shaw, was as Malthusian as Wells, but intervened to oppose Wells'sWeismannism. An ardent Lamarckian Shaw ultimately reflected that Morris might have been right.

Müller received Darwin’s Power of Movement in Plants in 1881. In response, he sent Darwin new evidence for universal nutational movement in plants and some functions it served. In 1883 Müller’s younger half brother, Wilhelm, joined Fritz for two years, working primarily on larval butterflies, their food plants, and their phylogeny. Wilhelm’s letters home describe Fritz and Caroline’s family life. Starting in 1885, correspondence with August Weismann explored the continuity of the germplasm. Weismann’s account implied that novel characteristics can only be generated by sexual mixing of germplasms with one exception: unicellular organisms have no separate somatic cells. Therefore, environmental change could yield inheritance of acquired characteristics. Müller denied this restriction. Continuity of the germplasm does not guarantee its constancy nor block novel heritable variations. Indeed, asexually propagating plants inherit novel variations, even when the variation originated in differentiated somatic cells: buds or runners occasionally transmit novel heritable characters to their offspring.

Political Descent closes with a consideration of the young socialist statistician, Karl Pearson. Pearson debated the extent to which Weismann's views had implications for socialist politics. In the pages of Nature as well as in more popular journals he rejected the views of the popular author, Benjamin Kidd, who had repeated the argument that Weismann undermined socialism in his bestselling Social Evolution. Even though Pearson eventually came to accept Weismann's views, he argued that a combination of eugenics at home and an imperialist foreign policy would maintain a sufficiently Malthusian struggle to prevent national degeneration. The onset of the Great War of 1914 is a logical place to bring this history to a close. It was in the wake of the horrors of the First World War that Darwinian arguments went out of favor in British political circles, even though they were picked up again in later years.

This book has examined the texts, the translation process, and especially the intellectual context in which Charles Darwin’s On the Origin of Species was written and interpreted. The result is a very different picture, not only of Heinrich Georg Bronn and Ernst Haeckel but also of the older morphology with which they were associated. This new picture of Haeckel has implications for our understanding of later developments in evolution and evolutionary thought and sheds new light on his conflicts over “mechanistic” and experimental approaches to embryology, particularly with Wilhelm His or Wilhelm Roux, and with August Weismann. Haeckel’s dispute with Weismann to define and defend Darwinism persisted into the twentieth century.

In the late 1800s, Charles Darwin and other naturalists supported a blending view of inheritance whereby offspring possess a middling mix of their parents' traits. Many of these naturalists also argued that individuals pass at least some of their acquired characteristics to their descendants. Darwin proposed that acquired characteristics and other environmentally induced changes in a parent's hereditary material (which he called “gemmules”) account in large part for the inheritable variations that drove evolution. Inspired by the evolutionary theories of his first cousin, Darwin, Francis Galton developed hereditarian notions that helped to lay the foundation for both genetics and eugenics. Eugenics was endorsed by evolutionary geneticists such as August Weismann, Karl Pearson, W. F. R. Weldon, William Bateson, and Hugo de Vries, which, as a result, gave it enormous scientific credibility in America and Europe. This chapter explores the role of biology in the emergence of the eugenics movement in the Anglo-Saxon world.

This chapter presents three important reasons why evolutionary biologists dismissed Lamarckism or soft inheritance from the Modern Synthesis. The first is the legacy of adhering to August Weismann's ideas of the separation of germ line and soma. The second is the absence, from the 1920s through the 1940s, of convincing for strong evidence for the inheritance of acquired characteristics. The third reason came from the assumptions of population and quantitative genetics, which were developing at the time.

In reaction to Darwin’s On theOrigin of Species, which he read in October 1861, Müller reorganized his ongoing research on crustacea. One result was his Für Darwin, published in 1864, utilizing detailed crustacean research. Müller applied Darwin’s theory of natural selection to crustaceans, yielding some novel predictions, some of which he subsequently verified. Müller showed that crustacean embryology provided a partial guide to crustacean genealogy, demonstrated that genealogical thinking resolved puzzles in the chaotic taxonomy of crustaceans, defended Darwin’s theory against several common objections, and demonstrated its superiority to special creationist alternatives. Darwin responded to Für Darwin with high praise and instigated an English translation of the book. He initiated correspondence with Müller, in which they discussed applications and extensions of Darwin’s theories and directions for further fieldwork. Für Darwin initially drew a generally tepid reception, with severe reviews from some critics. The chapter closes by comparing Müller’s evaluations of Darwin’s theory with those of Ernst Haeckel and August Weismann.