DEPARTMENT OF EXPERIMENTAL EVOLUTION. 133 



sexual forms. Practically all workers with Cladocera (except the 

 genus Simocephalus, in one case) who have reached a conclusion on 

 the matter are in accord with Keilhack's statement. 



In Dr. Banta's laboratory, however, quite a different result has been 

 gained. Two Simocephalus lines have been reared for three years, 

 during which time reproduction has been solely by parthenogenesis for 

 123 generations, and, indeed, no males have appeared. Another species, 

 a long-spined species of Daphnia, has been kept reproducing by par- 

 thenogenesis only for 87 generations. Lines of Daphnia culex have 

 been carried for nearly four years, and in several lines for from 148 to 

 157 generations, solely by parthenogenesis and without the production 

 of males. In all of these cases there is no sign of loss of vigor in the 

 stock. Periods of depression have, indeed, occurred, but these have 

 always been obviously due to poor food conditions. As a test for 

 possibly reduced vigor, careful comparisons between the hues reproduc- 

 ing for nearly 150 generations parthenogenetically and ''wild" lines 

 recently brought under laboratory conditions have been made through- 

 out a period of several months. Three bases of comparison were 

 used: the size of the first brood a mother produces, the age at which 

 the first brood is produced, and the interval between the production 

 of the first and second broods. The results of comparison on each of 

 these points show that the young mothers in lines which have long 

 reproduced parthenogenetically produce as large first broods, produce 

 them at as early an age, and produce a second brood as quickly, as the 

 young mothers in "wild" lines recently brought in from natural ponds. 

 Dr. Banta concludes that, since no diminution in vigor or need for 

 sexual reproduction is apparent in the different species after 87, 124, 

 and 157 generations, respectively, sexual reproduction and the sexual 

 cycle in these Cladocera are probably not inherently necessary, but 

 are consequences of certain special environmental conditions. 



That such special conditions may induce the production of males has, 

 indeed, been demonstrated for rotifers by Whitney; and observations 

 made by Dr. Banta show the same thing in Cladocera. In the "long 

 spine" Daphnia one gynandromorph and a few males in two successive 

 generations in two strains appeared at a time when males of other 

 species of Cladocera (Daphnia pulex and Moina) were extremely 

 abundant in the pond from which were obtained the food and water 

 in which these cultures were maintained. The fact that a few males 

 appeared in two strains reared under laboratory conditions simul- 

 taneously with the appearance of numerous males of two other species 

 living in the outdoor pond which furnished the culture water strongly 

 suggests the environmental control of the production of males. 



