Editing August Weismann (section)

== Contributions to evolutionary biology ==
At the beginning of Weismann's preoccupation with evolutionary theory was his grappling with Christian [[creationism]] as 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 ===
[[File:Weismann's Germ Plasm.svg|thumb|upright=1.5|Weismann's [[germ plasm]] theory. The hereditary material, the germ plasm, is transmitted only by the [[gonad]]s. Somatic cells (of the body) [[embryology|develop afresh]] in each generation from the germ plasm.]]

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 Wilhelm Gottfried von Waldeyer-Hartz|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,<ref>[http://carnegieinstitution.org/cover/top_100/ 100 Greatest Discoveries – Carnegie Institution] {{webarchive|url=https://web.archive.org/web/20070927070422/http://carnegieinstitution.org/cover/top_100/ |date=2007-09-27 }} at carnegieinstitution.org</ref> and one of the 10 most important discoveries in [[cell biology]].<ref>[http://science.discovery.com/convergence/100discoveries/big100/biology.html The Science Channel :: 100 Greatest Discoveries: Biology] {{webarchive|url=https://web.archive.org/web/20061024155730/http://science.discovery.com/convergence/100discoveries/big100/biology.html |date=2006-10-24 }} at science.discovery.com</ref>

Meiosis was discovered and described for the first time in [[sea urchin]] [[egg (biology)|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.<ref>Although, of course, [[Ernst Haeckel]] had; but he was not a cytologist.</ref> All this took place before the rediscovery of the work of Mendel. 

=== 1868–1881/82 ===
Weismann started out believing, like many other 19th century scientists, among them [[Charles Darwin]], that the observed variability of individuals of one species is due to the inheritance of ''sports'' (Darwin's term). He believed, as written in 1876, that transmutation of species is directly due to the influence of environment. He also wrote, "if every variation is regarded as a reaction of the organism to external conditions, as a deviation of the inherited line of development, it follows that no evolution can occur without a change of the environment". (This is close to the modern use of the concept that changes in the environment can mediate selective pressures on a population, so leading to evolutionary change.) Weismann also used the classic [[Lamarckism|Lamarckian]] metaphor of use and disuse of an organ.

=== 1882–1895 ===
Weismann's first rejection of the [[inheritance of acquired traits]] was in a lecture in 1883, titled "On inheritance" ("Über die Vererbung"). Again, as in his treatise on creation vs. evolution, he attempts to explain individual examples with either theory. For instance, the existence of non-reproductive castes of ants, such as workers and soldiers, cannot be explained by inheritance of acquired characters. [[Germ plasm theory]], on the other hand, does so effortlessly. Weismann used this theory to explain Lamark's original examples for "use and disuse", such as the tendency to have degenerate wings and stronger feet in domesticated waterfowl.

=== 1896–1910 ===
Weismann worked on the embryology of sea urchin eggs, and in the course of this observed different kinds of cell division, namely equatorial division and reductional division, terms he coined (''Äquatorialteilung'' and ''Reduktionsteilung'' respectively).

His ''germ plasm theory'' states that multicellular organisms consist of [[germ cell]]s containing heritable information, and [[somatic cells]] that carry out ordinary bodily functions. The germ cells are influenced neither by environmental influences nor by learning or morphological changes that happen during the lifetime of an organism, which information is lost after each generation. The concept as he proposed it was referred to as ''Weismannism'' in his day, for example in the book ''An examination of Weismannism'' by [[George Romanes]]<ref name="GJR"/> This idea was illuminated and explained by the rediscovery of [[Gregor Mendel]]'s work in the early years of the 20th century (see [[Mendelian inheritance]]).

=== Experiments on the inheritance of mutilation===
The idea that germline cells contain information that passes to each generation unaffected by experience and independent of the somatic (body) cells, came to be referred to as ''the Weismann barrier'', and is frequently quoted as putting a final end to the theory of [[Lamarck]] and the inheritance of acquired characteristics. What Lamarck claimed was the inheritance of characteristics acquired through effort, or will.

Weismann conducted the experiment of removing the tails of 68 white mice, repeatedly over 5 generations, and reporting that no mice were born in consequence without a tail or even with a shorter tail. He stated that "901 young were produced by five generations of artificially mutilated parents, and yet there was not a single example of a rudimentary tail or of any other abnormality in this organ."<ref>{{cite book |last=Tollefsbol |first=Trygve |title=Handbook of Epigenetics: The New Molecular and Medical Genetics |url=https://books.google.com/books?id=uJupDQAAQBAJ&pg=PA234 |year=2017 |publisher=Elsevier Science |isbn=978-0-12-805477-2 |page=234}} Originally published in Weismann's 1889 [http://www.esp.org/books/weismann/essays/facsimile/ Essays Upon Heredity].</ref> Weismann was aware of the limitations of this experiment, and made it clear that he embarked on the experiment precisely because, at the time, there were many claims of animals inheriting mutilations (he refers to a claim regarding a cat that had lost its tail having numerous tail-less offspring). There were also claims of Jews born without foreskins. None of these claims, he said, were backed up by reliable evidence that the parent had in fact been mutilated, leaving the perfectly plausible possibility that the modified offspring were the result of a mutated gene. The purpose of his experiment was to lay the claims of ''inherited mutilation'' to rest. The results were consistent with Weismann's germ plasm theory.