Pangenesis was Charles Darwin's hypothetical mechanism for heredity. He presented this 'provisional hypothesis' in his 1868 work The Variation of Animals and Plants under Domestication and felt that it brought 'together a multitude of facts which are at present left disconnected by any efficient cause'. The etymology of the word comes from the Greek words pan (a prefix meaning "whole", "encompassing") and genesis ("birth") or genos ("origin"). The hypothesis was eventually replaced by Mendel's laws of inheritance.
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The pangenesis theory, similar to Hippocrates's views on the topic, imply that the whole of parental organisms participate in heredity (thus the prefix pan) while adapting to cell theory. Much of Darwin's model was speculatively based on inheritance of tiny heredity particles he called gemmules that could be transmitted from parent to offspring. Darwin emphasized that only cells could regenerate new tissues or generate new organisms. He posited that atomic sized gemmules formed by cells would diffuse and aggregate in the reproductive organs.
Early history
The hypothesis of pangenesis was developed by the ancient Greek philosophers such as Hippocrates and Democritus. Darwin wrote that Hippocrates' pangenesis was "almost identical with mine—merely a change of terms—and an application of them to classes of facts necessarily unknown to the old philosopher."
Historian of science Conway Zirkle has noted:
The hypothesis of pangenesis is as old as the belief in the inheritance of acquired characters. It was endorsed by Hippocrates, Democritus, Galen (?), Clement of Alexandria, Lactantius, St. Isidore of Seville, Bartholomeus Anglicus, St. Albert the Great, St. Thomas of Aquinas, Peter of Crescentius (?), Paracelsus, Jerome Cardan, Levinus Lemnius, Venette, John Ray, Buffon, Bonnet, Maupertius, von Haller and Herbert Spencer.
A very similar hypothesis to Darwin's pangenesis was held by the French naturalist Georges-Louis Leclerc, Comte de Buffon who developed a system of heredity. Commenting on Buffon's views, Darwin stated, "If Buffon had assumed that his organic molecules had been formed by each separate unit throughout the body, his view and mine would have been very closely similar."
In 1801, Erasmus Darwin advocated a hypothesis of pangenesis in the third edition of his book Zoonomia.
The Irish physician Henry Freke in his book Origin of Species by Means of Organic Affinity (1861) developed a variant of pangenesis.
In 1864, Herbert Spencer in his book Principles of Biology, proposed a theory of "physiological units". These hypothetical hereditary units were similar to Darwin's gemmules.
Overview
Darwin's pangenesis theory was complex as he tried to explain the process of sexual reproduction, passing of traits and complex developmental phenomena, such as cellular regeneration.
"As Darwin explained it, pangenesis was the highly abstract notion that every tissue, cell and living part of an organism produced minute, unseen gemmules (or what he sometimes called granules or germs) which carried inheritable characteristics and were transmitted to the offspring via the reproductive process. He was careful to specify that each part of an organism produced only information about itself. There were gemmules for hands and feet, not for whole organisms. Individual gemmules did not contain a complete microscopic blueprint for an entire creature in the way that Herbert Spencer or Carl von Nagel described. When the gemmules from each parent mixed in the foetus they would produce a unique new individual."Some gemmules remained dormant for generations, whereas others were routinely carried by all offspring. He thought about these literally, "almost as if gemmules were letters in the postal system". Every child was built up from a 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.". 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.
Hugo de Vries characterized the theory in 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.Darwin's pangenesis theory was criticised for its Lamarckian premise that parents could pass on traits acquired in their lifetime. Lamarckism fell from favour after August Weismann's research in the 1880s indicated that changes from use (such as lifting weights to increase muscle mass) and disuse (such as being lazy and becoming scrawny) were not heritable. Some Lamarckian principles, however, have not been entirely discounted and some of Darwin's pangenesis principles (in this regard) do relate to heritable aspects of phenotypic plasticity, while the status of gemmules has been firmly rejected. Darwin himself had noted that "the existence of free gemmules is a gratuitous assumption"; by some accounts in modern interpretation, gemmules may be considered a prescient mix of DNA, RNA, proteins, prions, and other mobile elements that are heritable in a non-Mendelian manner at the molecular level.
Galton's experiments on rabbits
Darwin's half-cousin Francis Galton conducted wide-ranging inquiries into heredity which led him to refute Charles Darwin's hypothetical theory of pangenesis. In consultation with Darwin, he set out to see if gemmules were transported in the blood. In a long series of experiments in 1869 to 1871, he transfused the blood between dissimilar breeds of rabbits, and examined the features of their offspring. He found no evidence of characters transmitted in the transfused blood.
Darwin challenged the validity of Galton's experiment, giving his reasons in an article published in 'Nature' where he wrote: "Now, in the chapter on Pangenesis in my Variation of Animals and Plants under Domestication, I have not said one word about the blood, or about any fluid proper to any circulating system. It is, indeed, obvious that the presence of gemmules in the blood can form no necessary part of my hypothesis; for I refer in illustration of it to the lowest animals, such as the Protozoa, which do not possess blood or any vessels; and I refer to plants in which the fluid, when present in the vessels, cannot be considered as true blood." He goes on to admit: "Nevertheless, when I first heard of Mr. Galton's experiments, I did not sufficiently reflect on the subject, and saw not the difficulty of believing in the presence of gemmules in the blood."