4Ub THE AMERICAN NATURALIST [Vol. XLIV 



started: at 4-10°, 19-20° and 29-31° C. The results 

 were as follows : 











4-10° 



22 





35 days 

 14 " 



19-21° 

 29-31° 



20 

 17.7 



0.25 



All were given a surplus of food daily. It may be 

 observed that a higher temperature has the same effect 

 as insufficient food. 



Salts. — Since salts have such a marked effect on the 

 development of marine and some fresh-water animals, 

 I placed daphnids in the strongest solutions of various 

 salts that they would live in (without acclimatization). 

 The effects in two months (four generations) were un- 

 noticeable. 



Light. — Cultures were kept in the dark and in diffuse 

 and direct sunlight, but no effect was observed. 



A number of observers have recorded season-poly- 

 morphism in daphnids. Wesenberg-Lund 4 pointed out 

 that when the specific gravity and consequent buoyancy 

 of the water decreased — by heat in summer — the body 

 of the daphnids became smaller or were provided with 

 outgrowths, so as to offer a greater resistance to sinking. 

 Wolfgang Ostwald 5 produced, all at the same time, the 

 forms that occurred in nature at different seasons, by 

 varying the temperature. He emphasized the fact that 

 rise in temperature lowered the internal viscosity of the 

 water. He found that in the warm cultures the daphnids 

 often became productive at an undeveloped stage and, 

 as is true generally, reproduction retarded body growth. 



4 "TJeber das Abhangigkeitsverhaltnis zwischen den Ban der Plankton- 

 organismen und den spoeifis.-h.-n C^vi.-ht -I.-s Siisswassers, " Biol. Centlb., 



