GENETIC CHANGES 213 



phalus, Daphnia) with a view to detecting genetic changes, 

 if such occur. His attention has been centered upon the 

 characters which distinguish females (the ordinary partheno- 

 genetic individuals) from the more rare males. He has ob- 

 served the occurrence as mutations in Simocephalus vetulus 

 and in several different strains of Daphnia longispina, of 

 what are called "sex intergrades," individuals intermediate 

 in character between males and females as regards the sex- 

 differentiating characters both primary and secondary, or 

 showing various combinations of the several characters 

 which ordinarily distinguish the sexes. ^ That these varia- 

 tions are due to real genetic changes is shown by their occur- 

 rence in parthenogenetic lines descended (asexually) from a 

 common mother individual; that their occurrence is not rare 

 is shown by the fact that five out of six lines of Daphnia 

 under observation in the year 1918 were observed to give 

 rise to strains of sex intergrades. Further, such changes did 

 not occur in single lines once only and cease thereafter. Six 

 lines were propagated from the descendants of a single mu- 

 tant sex intergrade,and selected, three toward normal female- 

 ness, three toward maleness. The selection is characterized 

 as "somewhat effective." "In most later generations," says 

 Banta, "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 in- 

 tergrade. ... In general there is a fairly pronounced dif- 

 ference between the characters of the stock in strains selected 

 toward femaleness and in strains selected toward a more 



' Banta enumerates five easily recognized secondary sex-characters in Daphnia. 

 See Fig. 187. These are (1) brnly .size, greater in females than in males; (2) outline 

 of the head, forming a beak in the female but not in the male; (3) size and character 

 of the first antenna, well developed in males but rudimentary in females; (4) outline 

 and hairiness of ventral anterior margin of carapace, which in males forms almost a 

 right angle and is hairy, but in females is rounded and hairless; (.5) character of 

 first thoracic appendage, in males with a hook-shaped finger-like projection, in" fe- 

 males without hook and branched into many long terminal filaments. 



