The variation of animals in nature / by G.C. Robson and O.W. Richards.

  • Guy Coburn Robson
Date:
1936
Catalogue details

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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    6 THE VARIATION OF ANIMALS IN NATURE difiference between the two ; both marked and slight variations are known to be heritable. (c) (i) Variant individuals are not distributed in space at random and in a chaotic fashion. In the first place there is a very marked correlation (more marked in some groups of animals than in others) between the ecological background and the type of variation, which is one of the most obvious effects of the susceptibility of the living organism to its environ¬ ment. There is also a tendency for variant individuals which demonstrably do not owe their peculiarities to their environment to be distributed in certain specific ways. The most familiar example of such distribution is the geographical race. (ii) The frequency of heritable variation is one of the most important topics of modern evolutionary study. It is now generally agreed that gene-mutations are of the greatest im¬ portance, as they are regarded as the only source of new evolutionary material. It is usually stated that they occur very infrequently, and this conception is of prime importance in the modern statement of the theory of Natural Selection (Fisher, 1930 ; Haldane, 1932). How true this conception is it is impossible to say, as the subject has only been intensively studied in two species kept in artificial conditions. However, it is desirable to keep in mind the probability that the very great profusion of variation among animals in nature is due mainly to somatic differences and factorial recombinations. This conception has introduced a rather different outlook on the rôle of Natural Selection. Darwin in no place in ' The Origin ' or any other of his works, as far as we know, committed himself to any pronouncement as to the frequency of heritable variation. He repeatedly insisted indeed on the slowness of the selective process. This we imagine was due to his belief in ' blended inheritance ' and his realisation of the smallness of the individual steps and the comparative infrequency of serviceable ones, rather than to any idea of the infrequency of any heritable variation. Nevertheless he conveys the distinct impression that he thought that the stock of heritable variation was plentiful. We are now confronted with the suggestion that a/i)' kind of mutation is very rare, so that the additional qualification that it must also be serviceable renders it highly necessary that Selection must act with great efficiency ; it also introduces the question—how frequently will such rare mutations coincide with the selective circumstances
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