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BIOGENESIS (from the Gr. (3ios, See also: term for the theory according to which each living organism, however See also: simple, arises by a See also: process of budding, fission, spore-formation of sexual See also: reproduction from a See also: parent organism
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Under the heading of See also: ABIOGENESIS (q.v.) is discussed the series of steps by which the See also: modern acceptance of biogenesis and rejection of abiogenesis has been broughtabout
.
No biological generalization rests on a wider series observations, or has been subjected to a more critical See also: scrutiny than that every living organism has come into existence from a living portion or portions of a pre-existing organism
.
In the articles REPRODUCTION and See also: HEREDITY the details of the relations between parent and offspring are discussed
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There remains for treatment here a curious collateral issue of the theory
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It is within See also: common observation that parent and off-spring are alike: that the new organism resembles that from which it has come into existence: in See also: fine, biogenesis is homogenesis
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Every organism takes origin from a parent organism of the same kind
.
The conception of homogenesis, however, does not imply an absolute similarity between parent and organism
.
In the first place, the normal See also: life-See also: cycle of See also: plants and animals exhibits what is known as alternation of generations, so that any individual in the chain may resemble its See also: grand-parent and its grand-See also: child, and differ markedly from its parent and child
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Next, any organism may pass through a series of See also: free-living larval stages, so that the new organism at first resembles its parent only very remotely, corresponding to an early stage in the life-See also: history of that parent
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(See See also: EMBRYOLOGY, LARVAL FORMS and REPRODUCTION.) Finally, the conception of homogenesis does not exclude the differences between parent and offspring that continually occur, forming the material for the slow alteration of See also: stocks in the course of See also: evolution (see VARIATION AND SELECTION)
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Homogenesis means simply that such organism comes into existence directly from a parent organism of the same See also: race, and hence of the same See also: species, sub-species, genus and so forth
.
From See also: time to time there have been observers who have maintained a belief in the opposite theory, to which the name heterogenesis has been given
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According to the latter theory, the offspring of a given organism may be utterly different from itself, so that a known animal may give rise to another known animal of a different race, species, genus, or even See also: family, or to a plant, or See also: vice versa
.
The most extreme cases of this belief is the well-known See also: fable of the " See also: barnacle-geese," an illustrated account of which was printed in an early See also: volume of the Royal Society of See also: London
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Buds of a particular See also: tree growing near the See also: sea were described as producing barnacles, and these, falling into the See also: water, were supposed to develop into geese
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The whole See also: story was an imaginary embroidery of the facts that barnacles attach themselves to submerged See also: timber and that a species of See also: goose is known as the bernicle goose
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In modern times the exponents of heterogenesis have limited themselves to cases of microscopic animals and plants, and in most cases, the observations that they have brought forward have been explained by minuter observation as cases of See also: parasitism
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No serious observer, acquainted with modern microscopic technical methods, has been able to confirm the explanation of their observations given by the few modern believers in heterogenesis
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