6 THE VARIATION OF ANIMALS IN NATURE 



difference 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 role 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 any 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 



