THE ORIGIN OF VARIATION 19 



evolution. An intelligent layman once observed to one of us : 

 ' Why do you worry how variations arise : surely it is their 

 fate that matters ? ' Up to a point this is a valid criticism. 

 But, if we anticipate what is discussed in later chapters, it is 

 of considerable importance to decide whether new variants 

 arise only in a few scattered individuals or whether in some 

 cases whole populations are changed simultaneously. In the 

 former case we have to explain how the rare variants spread. 

 Again, any factor seriously affecting the rate of mutation might 

 have some influence on the chance of establishment of mutants, 

 especially in a rare species. In fact, apart from its logical 

 value in completing the theory of evolution, some knowledge 

 as to the origin of variations is necessary to form any theory 

 at all. 



1. Fluctuations 



That animals are more or less ' plastic ' or modifiable by 

 the environment in their structure, reactions and physiological 

 properties and activities is a fact of general knowledge. 1 We 

 do not propose to describe the many and varied effects which 

 external factors produce. They have been sufficiently detailed 

 in a number of works, and the varying action of temperature, 

 salinity and other chemical factors, humidity, etc., is familiar 

 to most biologists. Surveys of the subject have been made by 

 Hesse (1924), Cuenot (1925), and others, and studies of the 

 effects of all known environmental factors on a single group of 

 animals have been made for the Mollusca by Pelseneer (1920) 

 and less fully for the Insecta by Uvarov (1931) and Chapman 



(i93i)- 



In actual practice the proof of the non-hereditary nature 

 of a variation is relatively infrequent and the great bulk of 

 ' fluctuations ' is diagnosed as such on a priori grounds. Yet 

 no variation, as far as we know, declares its origin by its mere 

 ' appearance ' (p. 78). Whether it is a fluctuation or of fixed 

 heredity can be determined with certainty only by experiment. 

 Nevertheless many systematists and other writers proceed as if 

 it were possible to determine the nature of a variant by mere 



1 The ease with which some animals are experimentally or otherwise modified 

 by their environment should not lead us to ignore the marked constancy with 

 which others retain their specific characters. Nabours (1929, p. 55) lists a long 

 series of environmental factors which have no effect on the colour-patterns of the 

 grouse-locusts (Tettigidae). 



