The variation of animals in nature / by G.C. Robson and O.W. Richards.
- Guy Coburn Robson
- Date:
- 1936
Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
Credit: The variation of animals in nature / by G.C. Robson and O.W. Richards. Source: Wellcome Collection.
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![20 THE VARIATION OF ANIMALS IN NATURE inspection and write-off many forms as ' mere fluctuations ' or ' due to the environment.' It may be claimed that this procedure is justified by analogy with effects known to be produced by experiment. But actually a number of experi¬ ments has been claimed to show that certain effects are due to the environment, though no examination was made of the behaviour of the affected characters in heredity. Further, the amount of variation that is treated as non-heritable is far in excess of the number of cases that have been experimentally verified. It is not easy in fact to obtain more than relatively few instances of characters which have been shown experimentally to be non-heritable. Among the Mollusca, the form albo- lateralis of Arion empiricorum [ater] (Gollinge, 1909), the carinate and ecarinate forms oí Paludestrina jenkinsi (Robson, 1929), and various forms of Limnea peregra (Boycott, Oldham and Waters- ton, 1932) seem to be definitely fluctuations. Pelseneer (1920, p. 641) catalogues a list of 'variations non héréditaires' in the Mollusca ; but in all his cases, except that of Arion ater, there is no evidence that the character in question was not acting as a simple recessive, since the breeding test was not extended to more than one generation. In the insects, which have been so much used for genetical research, rather more cases are available. Some of the naturally occurring colour- variations of the bug Perillus bioculatus (Knight, 1924) and of the parasitic wasp Microbracon brevicornis (Génieys, 1922) are certainly not inherited. As for variations known only under artificial conditions, we may mention a white variant of the moth Ephestia kiihniella (Kühn and Henke, 1929) and a number of variants in Drosophila, especially reduplications of various organs (Morgan, Bridges and Sturtevant, 1925, p. 71 et seq.). Amongst birds, Beebe's (1907) experiments on the effect of a humid atmosphere on doves of the genus Scardafella are well known. In the rotifers, Kikuchi (1931) shows that in Brachionus pala lateral spines are developed when the animal is fed on the alga Scenedesmus ; the spines are lost when it is fed on Polytoma, and the action is completely reversible. A point worth remembering in discussing this question is that a given character may be heritable in one form and not in another. This is especially evident in the matter of the total size of an organism which is determined not only by the](https://iiif.wellcomecollection.org/image/b18020902_0043.JP2/full/800%2C/0/default.jpg)
