191'^] Michael: Behavior of Sal pa democratica 265 



Table 8 



Eeeoiistruction of table 4 by eliminating all hauls except those made 

 during June, 1909 



Solitary forms Aggregate forms 



Specimens Frequency Specimens Frequency 



Table 8 not only shows the same relation as does table 4, but 

 demonstrates that elimination of the seasonal effects due to the other 

 three months intensifies the difference between the abundance of soli- 

 tary forms in the colder water and their abundance in the warmer 

 water. In the colder water their abundance is reduced from 67 per 

 hour (table 4) to 56 per hour (table 8), while their abundance in the 

 warmer water is increased from 160 per hour (table 4) to 245 per 

 hour (table 8). Therefore, although advancing season and increasing 

 temperature were associated, the salpae were so distributed during 

 the months to which these data relate as to make it highly improbable, 

 if not impossible, for the relations revealed by tables 4 to 6 to have been 

 consequent upon the seasonal succession of the two generations. 



2. Relation Between Position and Temperature 



Another hypothesis has been raised. It is argued on the basis of 

 certain evidence relating particularly to aphids and rotifers that 

 high temperatures might favor production of solitary forms. Although 

 some of the more recent investigations (Whitney, 1907; Shull, 1911) 

 make such an effect of temperature doubtful, other causative factors, 

 such as substances in solution, may be so associated with temperature 

 as to lead to a greater production of solitary forms at high tempera- 

 tures. It is well, therefore, to consider this hypothesis. 



According to it the differentials established by tables 4 to 6 are 

 consequent not upon locomotion of solitary forms and aggregate forms, 

 nor upon their seasonal succession, but upon a stratification of sur- 

 face water into areas of high and low temperature, in the former of 

 which solitary forms predominate because of their greater production, 

 while aggregate forms predominate in the latter because of their 

 greater death rate in the warm areas. As in the ease of seasonal sue- 



