August Weismann

Trained in medicine, Weismann began teaching zoology and comparative anatomy at Freiburg in 1863 and became founding director of its zoological institute in 1867. Recurring eyesight problems restricted his microscopy, but he continued investigating butterflies, caterpillars and aquatic animals through experiments and theoretical writing.

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Weismann examined how temperature and other environmental conditions affected butterfly coloration and form. He also studied caterpillar markings, salamander metamorphosis and germ-cell formation in hydromedusae. Research on cladoceran water fleas included parthenogenesis, development from an unfertilized egg. These organisms supplied concrete problems behind his work on heredity.

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In an 1885 essay, Weismann argued that hereditary material passed between generations through germ cells. Changes acquired by somatic cells did not thereby enter the reproductive lineage. He used this distinction to challenge direct inheritance of bodily changes acquired during an individual’s lifetime.

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In Das Keimplasma, published in 1892, Weismann proposed hierarchical units called biophors, determinants, ids and idants to explain how hereditary material was organized and affected development. These units formed a historical theoretical model. His argument for continuity of hereditary transmission is distinguishable from his particular conjectures about its internal organization.

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