Editing Pangenesis (section)

==Theory==

=== Darwin ===

Darwin's pangenesis theory attempted to explain the process of [[sexual reproduction]], inheritance of traits, and complex [[developmental biology|developmental]] phenomena such as cellular [[Regeneration (biology)|regeneration]] in a unified mechanistic structure.<ref name="Geison69">{{cite journal |last1=Geison |first1=G. L. |year=1969 |title=Darwin and heredity: The evolution of his hypothesis of pangenesis |journal=J Hist Med Allied Sci |volume=XXIV |issue=4 |pages=375–411 |doi=10.1093/jhmas/XXIV.4.375 |pmid=4908353}}</ref><ref name="Jablonka05">{{cite book |last1=Jablonka |first1=E. |last2=Lamb |first2=M. |title=Evolution in four dimensions: Genetic, epigenetic, behavioural and symbolic |year=2005 |publisher=MIT Press |url=https://books.google.com/books?id=EaCiHFq3MWsC |isbn=978-0-262-10107-3}}</ref> Longshan Liu wrote that in modern terms, pangenesis deals with issues of "dominance inheritance, [[Grafting|graft hybridization]], [[Reversion (genetics)|reversion]], xenia, [[Telegony (inheritance)|telegony]], the inheritance of acquired characters, regeneration and many groups of facts pertaining to variation, inheritance and development."<ref name=":4"/> Mechanistically, Darwin proposed pangenesis to occur through the transfer of organic particles which he named 'gemmules.' Gemmules, which he also sometimes referred to as ''{{not a typo|plastitudes}},''<ref>Allaby, Michael. 2010. ''Animals: From Mythology to Zoology'' [https://books.google.com/books?id=KtKZ12YN9qcC&dq=plastitude+pangenesis&pg=PT201 "Plastitude"]</ref> pangenes, granules,<ref name=":0">{{cite book |title=Charles Darwin—The Power of Place |last=Browne |first=Janet |date=2002 |publisher=Jonathon Cape |location=London |page=275}}</ref> or germs, were supposed to be shed by the organs of the body and carried in the bloodstream to the reproductive organs where they accumulated in the germ cells or gametes.<ref name="Darwin68">{{cite book |last=Darwin |first=Charles |year=1868 |title=The Variation of Animals and Plants Under Domestication |location=London |publisher=[[John Murray (publishing house)|John Murray]] |url=http://darwin-online.org.uk/content/frameset?itemID=F880.1&viewtype=text&pageseq=1 |isbn=978-1-4191-8660-8}}</ref> Their accumulation was thought to occur by some sort of a 'mutual affinity.'<ref name="Geison69"/> Each gemmule was said to be specifically related to a certain body part- as described, they did not contain information about the entire organism.<ref name=":0"/> The different types were assumed to be dispersed through the whole body, and capable of self-replication given 'proper nutriment'. When passed on to offspring via the reproductive process, gemmules were thought to be responsible for developing into each part of an organism and expressing characteristics inherited from both parents.<ref name=":0"/> Darwin thought this to occur in a literal sense: he explained cell proliferation to progress as gemmules to bind to more developed cells of their same character and mature. In this sense, the uniqueness of each individual would be due to their unique mixture of their parents' gemmules, and therefore characters.<ref name=":0"/> Similarity to one parent over the other could be explained by a quantitative superiority of one parent's gemmules.<ref name=":4">{{Cite journal |last=Liu |first=Yongsheng |date=May 2008 |title=A new perspective on Darwin's Pangenesis |journal=Biological Reviews |volume=83 |issue=2 |pages=141–149 |doi=10.1111/j.1469-185x.2008.00036.x |pmid=18429766 |s2cid=39953275}}</ref> Yongshen Lu points out that Darwin knew of cells' ability to multiply by self-division, so it is unclear how Darwin supposed the two proliferation mechanisms to relate to each other.<ref name=":4"/> He did clarify in a later statement that he had always supposed gemmules to only bind to and proliferate from developing cells, not mature ones.<ref name=":5">{{Cite news |url=https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-6442.xml;query=gemmule;brand=default |title=To Scientific Opinion |last=Darwin |first=Charles |date=October 20, 1869 |work=Darwin Correspondence Project |access-date=2018-06-01}}</ref> Darwin hypothesized that gemmules might be able to survive and multiply outside of the body in a letter to [[Joseph Dalton Hooker|J. D. Hooker]] in 1870.<ref>{{Cite news |url=https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-7273.xml;query=multiply;brand=default |title=To J. D. Hooker |last=Darwin |first=Charles |date=July 12, 1870 |work=Darwin Correspondence Project |access-date=2018-06-01}}</ref>

Some gemmules were thought to remain dormant for generations, whereas others were routinely expressed by all offspring. Every child was built up from selective expression of the mixture of the parents and grandparents' gemmules coming from either side. Darwin likened this to gardening: a flowerbed could be sprinkled with seeds "most of which soon germinate, some lie for a period dormant, whilst others perish."{{sfn |Browne |2002 |p=276}} He did not claim gemmules were in the blood, although his theory was often interpreted in this way. Responding to [[Fleming Jenkin]]'s review of ''[[On the Origin of Species]]'', he argued that pangenesis would permit the preservation of some favourable variations in a population so that they wouldn't die out through blending.{{sfn |Browne |2002 |p=283}}

Darwin thought that environmental effects that caused altered characteristics would lead to altered gemmules for the affected body part. The altered gemmules would then have a chance of being transferred to offspring, since they were assumed to be produced throughout an organism's life.<ref name=":1"/> Thus, pangenesis theory allowed for the Lamarckian idea of transmission of characteristics acquired through use and disuse. Accidental gemmule development in incorrect parts of the body could explain deformations and the 'monstrosities' Darwin cited in ''Variation''.<ref name=":1"/>

=== De Vries ===
[[Hugo de Vries]] characterized his own version of pangenesis theory in his 1889 book ''Intracellular Pangenesis'' with two propositions, of which he only accepted the first:

:I. In the cells there are numberless particles which differ from each other, and represent the individual cells, organs, functions and qualities of the whole individual. These particles are much larger than the chemical molecules and smaller than the smallest known organisms; yet they are for the most part comparable to the latter, because, like them, they can divide and multiply through nutrition and growth. They are transmitted, during cell-division, to the daughter-cells: this is the ordinary process of heredity.
:II. In addition to this, the cells of the organism, at every stage of development, throw off such particles, which are conducted to the germ-cells and transmit to them those characters which the respective cells may have acquired during development.<ref>{{cite book |last=de Vries |first=Hugo |title=Intracellular Pangenesis |url=http://www.esp.org/books/devries/pangenesis/facsimile/ |access-date=May 2, 2015 |date=1910 |orig-year=1889 |page=63}}</ref>

=== Other variants ===

The historian of science [[E. Janet Browne|Janet Browne]] points out that while <!--Herbert--> Spencer and [[Carl von Nägeli]] also put forth ideas for systems of inheritance involving gemmules, their version of gemmules differed from Darwin's in that it contained "a complete microscopic blueprint for an entire creature."{{sfn |Browne |2002 |p=281}} Spencer published his theory of "physiological units" three years prior to Darwin's publication of ''Variation''.<ref name="Zirkle 1935"/> Browne adds that Darwin believed specifically in gemmules from each body part because they might explain how environmental effects could be passed on as characteristics to offspring.{{sfn |Browne |2002 |p=281}}

Interpretations and applications of pangenesis continued to appear frequently in medical literature up until [[August Weismann|Weismann's]] experiments and subsequent publication on germ-plasm theory in 1892.<ref name="Zirkle 1935"/> For instance, an address by Huxley spurred on substantial work by Dr. James Ross in linking ideas found in Darwin's pangenesis to the [[germ theory of disease]].<ref name=":7">{{Cite book |title=The Graft Theory of Disease: Being an Application of Mr. Darwin's Hypothesis of Pangenesis to the Explanation of the Phenomena of the Zymotic Diseases |last=Ross |first=James |publisher=Lindsay and Blakiston |year=1872 |location=Philadelphia}}</ref> Ross cites the work of both Darwin and Spencer as key to his application of pangenetic theory.<ref name=":7"/>