A PHYSIOLOGICAL CHARACTER. 135 



and, as with the Hnes of D. pulex, there is a striking resemblance 

 between the curves for any one of the strains and the composite 

 curve for all the corresponding strains of S. exspinosus. 



There is some tendency for the composite reaction-time curves 

 for strains of D. pulex to follow the same course as the curves for 

 S. exspinosus, but this is seen to a limited extent and in only a few 

 places, such as in a general rise for both species for June- July 1913 

 (figures IOd and 15) and in a general rise followed by a pronounced 

 drop in the period from April to November 1915 (figures IOd and 18). 

 While in general the physiology of these two species is much the same, 

 there are still such specific physiological differences that any coinci- 

 dental changes in reaction-times bespeak pronounced contempo- 

 raneous effects of environment. 



Seasonal Changes in Reactiveness. 



There is some tendency observable in the reaction-time curves 

 for a drop in the curves, indicating greater reactiveness during the 

 winter than the summer months. The composite curves for the D. 

 pulex lines (figure IOd) in general show lower portions of the curves 

 for the October-March periods, but this does not hold for the winter 

 period for 1913-1914. Seasonal changes are seen to some extent for 

 the winter and summer periods in the composite reaction-time 

 curves for the S. exspinosus lines (figure 18b) and in the six-month 

 curves for Lines 740 and 757 (figure 19). 



The seasonal (winter and summer) differences are possibly due 

 largely to temperature influences, but there is no certainty that this 

 seasonal shift is due to temperature alone or even to temperature as 

 indirectly working through the oxygen or other content of the water 

 used in the experimental tank and through the culture medium, 

 yet it seems probable that these influences may be a large factor in 

 these seasonal effects. 



Contemporaneous Shifts in Reaction-Time Means. 



Contemporaneous shifts in the reaction-time curves for the 

 strains of different lines of the same species are seen. There are a 

 few shorter time-periods in the data for which there are really re- 

 markable contemporaneous changes in the curves for the different 

 strains of the same species. For the August-September 1912 period 

 the 14 D. pulex strains, with two exceptions, showed drops in the 

 reaction-time curves. All but one of these drops were considerable 

 in extent. The two strains which did not show drops in this period 

 and the one which showed a slight drop in this period showed pro- 

 found drops in the next two-month period, while most of the strains 

 showed further drops in this later two-month period. Temperature 

 influences^ were not the factors determining these drops in the 



^Temperature is referred to frequently because the data include temperature records, whib 

 for the other environmental factors no definite measurements are available. 



