34 GENETICS 



plete pigmentation but by cutting up this continuous 

 series of variations into arbitrary groups of equal 

 extent, it was quite possible to arrange the data so 

 that they could be statistically treated just as con- 

 veniently as the integral variations mentioned above. 

 Groups or classes of this kind are termed graduated 

 variations. 



9. THE CAUSES OF VARIATION 



With respect to the causes of variation authoritative 

 biologists have taken different points of view. 



a. Darwin considered variations as axiomatic. An 

 axiom is self-evident, requiring no explanation. The 

 absence of variations in organisms rather than the oc- 

 currence of variations is, from this point of view, the 

 phenomenon requiring an explanation. Although Dar- 

 win himself spent some time in pointing out the univer- 

 sal occurrence of variability, he accepted it as a pri- 

 mary fact and proceeded from it as a starting point 

 without attempting to seek its causes. 



b. Lamarck and his followers have regarded the 

 causes of variation either as extrinsic, that is, refer- 

 able to external factors making up the environment of 

 the organism, or as intrinsic or physiological, that is, 

 based upon the efforts which an organism puts forth 

 to fit into its particular environment successfully. The 

 causes of variation are to be sought according to the 

 Lamarckian school, in the "environment" and "re- 

 sponse" sides of the triangle of life rather than in the 

 "heritage" side (Fig. 1). 



For example, Woltereck, by controlling the single 



