408 THE AMERICAN NATURALIST [ Vol. XLIV 



Effect of Environment on the Life Cycle 



In most species of daphnids, generations of partheno- 

 genetic females alternate with generations of males and 

 females which produce eggs that must be fertilized, and 

 either frozen, dried or kept a long time before they will 

 develop (resting or " winter" eggs). In different 

 species the number of successive parthenogenetic gen- 

 erations varies. In some all are, and in some none are, 

 parthenogenetic. 



I found that heat hastened the appearance of sexual 

 forms, as did starving or the accumulation of excretory 

 products. All of these factors might be combined in the 

 drying up of a pond, as heat would aid in drying, and 

 drying would concentrate the daplmids and their ex- 

 cretions, and concentration of the daphnids would cause 

 them to eat up the algae faster than they could multiply. 

 However, by keeping the culture cold, fresh or well-fed, 

 or all combined, I could delay but not prevent the ap- 

 pearance of sexual forms. 



Kurz 8 said the drying up of the water caused the ap- 

 pearance of sexual forms, and Schmankewitz 9 suggested 

 that it was the increase in salts. Weismann 10 tested 

 both of these hypotheses and concluded that they were 

 wrong. He also tried the effect of food and tempera- 

 ture, with varying results. He concluded that the life 

 cycle was fixed for each species and variety. Issako- 

 witz 11 concluded that cold favored the appearance of 

 sexual forms and warmth favored the parthenogenetic. 

 Also, hunger favored the appearance of sexual forms 

 and abundant food the parthenogenetic. It may be that 

 cold retarded multiplication of the food plant or the 



