August Weismann on Germ-Plasm Variation

Journal of the History of Biology 34 (3):517-555 (2001)
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.
Keywords August Weismann  development  evolutionary developmental biology  externalism  genetics  germ-plasm  heredity  inheritance of acquired characters  nineteenth century  sexual reproduction  variation
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    Frederick B. Churchill (2010). August Weismann Embraces the Protozoa. Journal of the History of Biology 43 (4):767 - 800.
    Mark A. Ulett (2014). Making the Case for Orthogenesis: The Popularization of Definitely Directed Evolution (1890–1926). Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences 45:124-132.
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