German zoologist (Potsdam 1834 - Jena 1919). A supporter of Darwinism, to which he had approached in 1860, and of a scientific materialism to which he gave the name of monism , he is best known for his general publications, whose informative principle is the theory of evolution, from "monera" to man, with genealogical trees in which imagination often makes up for the lack of data. He gave his name to the fundamental biogenetic law already expressed by E.-R.-A. Serres and F. Müller (according to which ontogeny recapitulates phylogeny), to which he attributed the value of the maximum proof of evolution and of which the theory of the gastrea is a corollary. Among his best-known works , Generelle Morphologie der Organismen (1866) should be mentioned.