a possible alternative. In his work Über die Berechtigung der Darwin'schen Theorie (On the justification of the Darwinian theory) he compared creationism and evolutionary theory, and concluded that many biological facts can be seamlessly accommodated within evolutionary theory, but remain puzzling if considered the result of acts of creation. After this work, Weismann accepted evolution as a fact on a par with the fundamental assumptions of astronomy (e.g. Heliocentrism). Weismann's position towards the mechanism of inheritance and its role for evolution changed during his life. Three periods can be distinguished. German work on cells Weismann's work on the demarcation between germ-line and soma can scarcely be appreciated without considering the work of (mostly) German biologists during the second half of the 19th century. This was the time that the mechanisms of cell division began to be understood. Eduard Strasburger, Walther Flemming, Heinrich von Waldeyer and the Belgian Edouard Van Beneden laid the basis for the cytology and cytogenetics of the 20th century. Strasburger, the outstanding botanical physiologist of that century, coined the terms nucleoplasm and cytoplasm. He said "new cell nuclei can only arise from the division of other cell nuclei". Van Beneden discovered how chromosomes combined at meiosis, during the production of gametes, and discovered and named chromatin. Walther Flemming, the founder of cytogenetics, named mitosis, and pronounced "omnis nucleus e nucleo" (which means the same as Strasburger's dictum). The discovery of mitosis, meiosis and chromosomes is regarded as one of the 100 most important scientific discoveries of all times, and one of the 10 most important discoveries in cell biology. Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by Oscar Hertwig. It was described again in 1883, at the level of chromosomes, by Van Beneden in Ascaris eggs. The significance of meiosis for reproduction and inheritance, however, was first described in 1890 by Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. Thus the work of the earlier cytologists laid the ground for Weismann, who turned his mind to the consequences for evolution, which was an aspect the cytologists had not addressed. All this took place before the work of Mendel had been rediscovered 1868–1881/82 Weismann started out believing, like many other 19th century scientists, among them Charles Darwin, that the observed variability