The Influence of Environment 297 



are not inherited, a fact contrary to the idea of Dar- 

 win, who assumed that by the selection of extreme 

 cases of fluctuating variation new varieties could de- 

 velop. What is the basis of this fluctuating variation? 

 The writer concluded that if fluctuating variations 

 were due to a slight variation in the quantity of a 

 specific substance — in some cases an enzyme — required 

 for the formation of a hereditary character, the tem- 

 perature coefficient might be used to test the idea. We 

 have just seen that the time required from insemina- 

 tion until the cell division of the first egg occurs is 

 very sharply defined for each temperature. If a large 

 number e,g, one hundred or more eggs are under obser- 

 vation simultaneously in a microscopic field it can be 

 seen that they do not all segment at the same time 

 but in succession; this is the expression of fluctuating 

 variation. Miss Chamberlain and the writer have 

 measured the time which elapses between the moment 

 the first egg of such a group segments and the moment 

 the last egg begins its segmentation, and found that this 

 latitude of variation is also very definite for each tem- 

 perature, and that its temperature coefficient is for 

 each interval of 10° practically identical with the 

 temperature coefficient of the segmentation for the 

 same interval.' The slight deviations are practically 

 all in the same sense and accounted for by a slight 

 deficiency in the nature of the experiments. The 

 ' Loeb, J., and Chamberlain, M. M., Jour. Exper. ZooL, 1915, xix., 559. 



