In the period 1875-1920, a debate about the generality and applicability of evolutionary theory to all organisms was motivated by work on unicellular ciliates like Paramecium because of their peculiar nuclear dualism and life cycles. The French cytologist Emile Maupas and the German zoologist August Weismann argued in the 1880s about the evolutionary origins and functions of sex (which in the ciliates is not linked to reproduction), and death (which appeared to be the inevitable fate of lineages denied sexual conjugation), (...) an argument rooted in the question of whether the ciliates and their processes where homologous to other cellular organisms. In the beginning of the twentieth century, this question of homology came to be less important as the ciliates were used by the British protozoologist Clifford Dobell and the American zoologist Herbert Spencer Jennings to study evolutionary processes in general rather than problems of development and cytology. For them, homology mattered less than analogy. This story illustrates two partially distinct problems in evolutionary biology: first, the question of whether all living things have common features and origins; and second, whether their history and current nature can be described by identical mechanisms. Where Maupas (contra Weismann) made the ciliates qualitatively the same as all other organisms in order to create a cohesive evolutionary theory for biology, Jennings and Dobell made them qualitatively different in order to achieve the same end. (shrink)

August Weismann is famous for having argued against the inheritance of acquired characters. However, an analysis of his work indicates that Weismann always held that changes in external conditions, acting during development, were the necessary causes of variation in the hereditary material. For much of his career he held that acquired germ-plasm variation was inherited. An irony, which is in tension with much of the standard twentieth-century history of biology, thus exists – Weismann was not a Weismannian. I distinguish three (...) claims regarding the germ-plasm: (1) its continuity, (2) its morphological sequestration, and (3) its variational sequestration. With respect to changes in Weismann’s views on the cause of variation, I divide his career into four stages. For each stage I analyze his beliefs on the relative importance of changes in external conditions and sexual reproduction as causes of variation in the hereditary material. Weismann believed, and Weismannism denies, that variation, heredity, and development were deeply intertwined processes. This article is part of a larger project comparing commitments regarding variation during the latter half of the nineteenth century. (shrink)

We began our survey at a time when Ehrenberg's functional principles concerning the design of all organisms prevailed in interpreting the taxonomic place and internal structure of Infusoria. Other options existed, such as Dujardin's sarcode theory and Siebold's cellular analogy, but these were not persuasive for reasons both relevant to and in addition to the microscopic observations. By mid-century other considerations, including the continuing search for complex life cycles and manifestations of sex, dictated the microscopist's rendering of infusorians. Müller and (...) his students saw spermatozoa and gonads; Stein envisioned alternating generations and generative nuclei; Balbiani found sexual behavior in the events of conjugation; and all interpreted the nuclei and nucleoli as gonads or gonadal products. Evolutionary theory provided a fourth dimension. Obsessed with phylogenies, particularly when joined with the germ-layer theory of development, Haeckel emphasized a structural interpretation and found a considerably altered cell theory ready to fill the breach.By the mid-1870s advances in cytology secured Siebold's original analogy. Infusorians now became single-celled organisms, the first in a long phylogenetic sequence that led to the “highest” metazoans. The analogy allowed Bütschli to invoke another biological principle, the physiological division of labor, to reinterpret the origin of the bisexual state. The ghost of Ehrenberg's polygastric theory had completely vanished. (shrink)

In a review of the cell biology and heredity studies of 1900–1910, Bernardino Fantini argues that the choice of an experimental subject or organism was crucial in opening up new discoveries and new theories for specific fields of research.69 Thinking on a broader level, Bütschli expressed a similar view when he stated that an understanding of the true nature and structure of the “elementary organism” was crucial to the whole of biology. In this article we have traced the impact of (...) Bütschli's unicellular model of protozoa up to the general acceptance of the eukaryotic cell as nature's primary organism. We have also seen that Bütschli's vision in the Studien gave birth to Protistenforschung and Zellforschung, sister sciences whose task it was to further and integrate knowledge of the living cell. Of necessity, we have confined ourselves to German scientific developments. However, our attempt to recapture and define the unity and meaning of a biology of the “cell-microcosm” has given us pause to reconsider the development and nature of Wilhelmian biological science. (shrink)