^91^1 Michael: Behavior of Salpa democratica 275 



Again, only one solitary form can be transported into cold surface 

 water by the aggregate salpae of each long chain, while several aggre- 

 gate forms must be transported into warm surface water by the soli- 

 tary salpa of each short chain. In other words, a decrease in number 

 of aggregate forms relative to an increase in temperature must be 

 masked by an increase in number of short chains to a far greater 

 extent than a decrease in number of solitary forms relative to a 

 decrease in temperature would be masked by an increase in number 

 of long chains. Therefore, aggregate forms should be more abundant 

 in the warmest surface w^ater than in that somewhat cooler. Indeed, 

 their abundance should approximately parallel that of chains, so that 

 a minimum ought to occur in water having a temperature about mid- 

 way between the two extremes, i.e., about 18?5C. Solitary forms, 

 however, being much less affected by the distribution of long chains 

 than aggregate forms are by that of short chains, should increase in 

 abundance as the temperature increases from its lowest, or nearly its 

 lowest, to its highest value. 



By retabulating the data with reference to three, four, five, and 

 more temperature groups, this locomotion theory will be subjected to 

 an empirical test. To sum up, the relations that according to theory 

 should be revealed are : 



1. Frequency of both generations nearly identical, and decreasing 

 with an increase in temperature to near the middle of its range, then 

 increasing with an increase in temperature. 



2. Frequency of both generations greater in the lowest than in the 

 highest temperature group, greater in the next lowest than in the next 

 highest, greater in the third lowest than in the third highest, and so on. 



3. Abundance of aggregate forms paralleling, or nearly paralleling, 

 frequency of both generations, appearing at its minimum relative to 

 medium temperatures. 



4. Abundance of solitary forms decreasing as the temperature 

 decreases throughout the entire range or nearly so ; if any increase in 

 abundance accompanies a decrease in temperature, this will be evi- 

 dent only in the coldest water when the number of temperature groups 

 is great enough to permit comparison of the effect of small ranges. 



In table 11 the data are arranged in three groups according as 

 the temperature varied from 15?9C to 17?4C, 17?5C to 19?0C, or 

 19?1C to 20?6C. The only haul (1579) made in water exceeding 

 20? 6C, and which did not capture a single individual of either genera- 

 tion, is excluded in order to preserve a uniform range in each group 

 of 1?6C. 



