DEPARTMENT OF EXPERIMENTAL EVOLUTION. 107 



As to his studies upon the selection in the sex-intergrade strains, 

 Dr. Banta reports as follows: 



"From 8 sex intergrades from the same cultm'e-bottle (the first undoubted 

 intergrades found in Daphnia longispina) , 8 intergrade strains were propa- 

 gated. Strains ii and vii were lost. The remaining 6 strains, after having 

 been propagated mthout reference to selection for two or three generations, 

 were subjected to selection. The basis of the selection was the degree of 

 intergradeness of the female intergrades chosen to propagate the strains. 

 Strains i, iv, and vi were used in an attempt to make them more strongly 

 intergrade through selection, and strains iii, v, and viii were selected to 

 cause them, if possible, to produce more nearly normal females. In general, 

 the selections have been somewhat effective. In most later generations the 

 stock in the strains selected away from the intergrade characters has been 

 moderately or only slightly intergrade, while in some cases the stock has been 

 almost wholly normal female. In the strains selected to make them strongly 

 intergrade the stock has usually been strongly intergrade. Environmental 

 conditions are effectual in influencing the stock, so that at times the strains 

 selected toward normal femaleness have been markedly intergrade, but in 

 general there is a fairly pronounced difference between the characters of the 

 stock in strains selected toward femaleness and in strains selected tov/ard a 

 more strongly intergrading condition." 



Dr. Banta has attempted little direct experimentation on sex-deter- 

 mination since his last report, as he has been a"waiting the arrival of 

 the Bovee potentiometer which is almost indispensable to the work. 

 He reports as follows upon the environment and sex determination in 

 Cladocera : 



"The earlier evidence, previously reported, consisted of the following: 



"(a) A simultaneous occurrence of males in two strains of Daphnia longi- 

 spina at a time when there were great numbers of males among the 'wild' 

 Daphnia pulex and Moina sp. in the pond from which the culture water 

 was obtained. (During almost four years of the history of the Cladocera 

 stock in the laboratory up to that time no males had occurred.) 



"(b) The ratios of the various sex forms produced in the several sex- 

 intergrade strains are frequently contemporaneously changed — the stock as 

 a whole (there being mothers of different ages and from different strains hut 

 all kept on the same food) at one time running strongly toward femaleness, 

 i. e., prevailing normal females and slightly intergrade females. At another 

 time the distribution of the normal females, female intergrades of various 

 degrees of maleness, male intergrades, and males is somewhat uniform. At 

 still another time the stock runs largely toward maleness, there being an 

 abnormally large percentage of males and male intergrades, and so few female 

 intergrades (or normal females) that at times some of the stock is in danger 

 of being lost from lack of females or female intergrades capable of producing 

 young. The influence of environmental factors alone explains such changes 

 in proportions of the female, male, and intergrade forms. 



"(c) In experiments with sex control as the object the experiments have 

 not all been successful, but males have usually appeared in smaller or larger 

 numbers in the culture-bottles in which they were expected, 



"(d) The striking 'epidemic' of males in the laboratory in May 1917, 

 when in the preceding 5| years so few males had appeared in the laboratory 

 stock, comes at a time when there were many males among the 'wild' stock 

 in the outdoor pond from which the culture-water was obtained, and so 

 added to the evidence that environmental influences are the causative agent 

 for the production of males. 



