120 CARNEGIE INSTITUTION OF WASHINGTON. 



return from high to low and back to high again, and a return from low to 

 high and back to low again. 



"The question arose as to whether these strains really possessed genetic 

 differences or whether their differences were merely of a cytoplasmic nature. 

 To test this, several high mothers from strain I were simultaneously given 

 the same treatment as an equal number of mothers of the same grade 

 from low strain III. The offspring of each set of mothers were carefully 

 graded. The result was as follows: the mothers of low strain III with a 

 weighted average grade of 24.9 produced young averaging grade 7.7, while 

 the mothers from high strain I, with a weighted average grade of 26.0, pro- 

 duced 3^oung averaging grade 25.3. Hence, while the mothers were of as 

 nearly equal grade as could readily be obtained, the offspring from the 

 mothers belonging to the low strain graded only one-third as high as offspring 

 from the high strain. This would seem to indicate clear genetic differences 

 between the two strains, and all the other evidence supports this conclusion. 



Another question that arose concerned the possibility of separate genetic 

 factors being involved in the development of the different secondary sex- 

 characters. This was tested by selecting different strains for different indi- 

 vidual secondary sex-characters. For example, in strain XVI individuals 

 were selected which had the most male-like antennules, while in respect to 

 other secondary sex-characters they were only moderately intergrade. The 

 result of selection on this basis for 10 generations and with 3 strains was 

 entirely negative. The secondary sex-characters are obviously transmitted 

 as a whole and not by separate factors for character of individual secondary 

 sex-characters — antennules, breast margins, etc. 



"Environmental influences affect the degree of intergradeness of secondary 

 sex-characters in sex-intergrade stock. At certain times all the stock is 

 relatively highly intergrade ; at other times it is all relatively less intergrade. 

 For example, in the 31st and 42d generations of selection are general uplifts 

 in the curves of intergradeness indicating increased degree of intergradeness 

 during those generations. In the 53d generation a general dip in the curves 

 followed by a rise in the next generation indicates lessened followed by 

 increased degree of intergradeness. These simultaneous fluctuations are 

 reflections of environmental factors." 



Sex-Intergrades Induced by Parasitism. 



Sex-intergrades are found in Crustacea that are parasitized and 

 rendered sterile. Such parasitized Crustacea have been described 

 by Giard (188G-88) in the case of several higher Crustacea and by 

 GeofTrey Smith (1910) in the spider crab Inachus, parasitized by Sac- 

 culina. During this year this phenomenon has been found by Dr. 

 Banta in the "new form" of Daphnia pulex obtained near Cold Spring 

 Harbor. He reports: 



"The secondary sex-characters in D. pulex are very similar to those of D. 

 longispina, which are figured in Year Book No. 16 for 1917, page 123. In 

 these parasitized individuals Dr. Banta found that the sex-intergrade con- 

 dition varied from only slightly intergrade to rather highly intergrade in 

 character. If a larger series could have been secured it seems probable 

 that most of the features of the D. longispina intergrade stock would have 

 appeared. Enough was seen to make it clear that (1) in the characters 

 affected, (2) the range of degree of intergradeness in difTerent individual 

 intergrades, and (3) the lack of uniformity in degree of intergradeness in the 



