VARIATION 



53 



The absence of variations in organisms rather than 

 the occurrence of variations is, from this point of 

 view, the phenomenon requiring an explanation. 

 Although Darwin himself spent some time in point- 

 ing out the universal occurrence of variability, he 

 accepted it as a primary 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 success- 

 fully. The causes of variation are to be sought ac- 



t 



■oO 





-L. 



-L. 



Af. 



M, 



t o/ 



50 35 '^0 45 50 55 60 65 70 75 80 85 90 95 100 /o 



< Ratio of height of head to length of shell > 



Fig. 29. — Schematic curve of the head height of Hyalodaphnia under 

 various conditions of nourishment. Adapted from Woltereck. 



cording to the Lamarckian school, in the "environ- 

 ment" and "training" sides of the triangle of life 

 rather than in the "heritage" side (Fig. l)c 



For example, Woltereck, by controlling the single 



