296 The Influence of Environment 



the variation from 7 or more to 2.2 for Qjo for the rate of 

 segmentation. 



Kanitz^ has shown that in a graph in which the 

 logarithms of the segmentation velocities are drawn 

 as ordinates and the temperatures as abscissas the 

 logarithms form two straight lines which are joined at 

 an angle. According to the law of van't Hoff and 

 Arrhenius concerning the influence of temperature upon 

 velocities of chemical reactions the logarithms should 

 lie in a straight line. We are dealing therefore in these 

 cases with two exponential curves, one representing in 

 Arhacia the interval 7-13° and the second from 13-26°; 

 in Strongylocentrotus between 3-9^ and 9-20°. 



It was found in these experiments that if measure- 

 ments of the Qio of later stages of development are 

 attempted the variations due to unavoidable difficulties 

 become too great to permit an equal degree of reliabihty 

 in the determinations. 



The vast importance of this influence of temperature 

 upon the rate of development is seen in the fact that in 

 addition to the food supply the rate of the maturing 

 of plants and animals depends on this factor. 



2. This influence of temperature upon develop- 

 ment has been used to find the conditions determining 

 fluctuating variation. The reader knows that by this 

 expression are understood the differences between in- 

 dividuals of a pure strain or breed. These variations 



* Kanitz, A., loc. cit., p. 123. 



