A PHYSIOLOGICAL CHARACTER. 153 



phototropism of other types of Drosophila, but these were associated 

 with differences in the color or other conditions of the eye itself. In 

 the tan mutant, on the other hand, no pecuharities could be dis- 

 covered in the eye. 



The writer has considerable (unpublished) data tending to show 

 that the amphipod Eucrangonyx gracilis, living in the surface streams 

 near Bloomington, Indiana, is less negatively phototropic than the 

 same species which lives in Mayfield's Cave, near at hand. The 

 differences in reaction are not large and consistent enough to be 

 very conclusive. In this species those inhabiting caves and those 

 inhabiting the open are indistinguishable morphologically, except for 

 differences in pigmentation, the former having fully pigmented eyes, 

 but no body pigment. 



In asexually reproducing forms^ Jennings (1908, 1909) with 

 Paramecium, Agar (1913, 1914) with Cladocera, Ewing (1914) with 

 aphids, Hanel (1908) and Lashley (1915, 1916) with Hydra, and 

 other workers have failed to obtain hereditarily diverse strains by 

 selection within the uniparental progeny of single individuals. 



On the other hand, Middleton (1915) in Stylonychia, Stout (1915) 

 with Coleus, Jennings (1916) with Difflugia, Root (1918) in Centro- 

 pyxis, and Hegner (1918, 1919) with Arcella seem to have found 

 genetic variation wherever they sought it. 



Stocking (1915), studying abnormahties in Paramecium, found 

 that of 122 strains containing abnormal individuals, in which the 

 character was clearly hereditary, 97 were persistently abnormal 

 races, i. e., they presumably were not subject to genetic variation 

 in this regard; and 25 strains were subject to genetic variation, i. e., 

 selection was effective in 25 strains. That is to say, about 20 per cent 

 of the strains studied by her, in which the character was inherited, 

 were subject to genetic variation. 



The results of selection with Protozoa are open to certain objec- 

 tions which Jennings (1916, p. 523) outlines somewhat as follows: 

 (1) that the results may be due to cytoplasmic influences and hence 

 may not be of real genetic significance; (2) that the result may be 

 due to nuclear recombination between the active nucleus and nuclear 

 material within the cytoplasm (chromidia); and (3) that nuclear 

 recombination may occur between different nuclear masses in the 

 same individual. Professor Jennings (1916, p. 525) believes that 

 these possibilities have ''no claim to greater probability than their 

 negatives, so far as our knowledge of the facts is concerned." The 

 opinion of different workers as to the validity of these objections 

 differs greatly. 



^In view of the many excellent historical accounts which have appeared it does not seem 

 desirable to review any considerable portion of the vast amount of work on selection within the 

 pure line. Excellent discussions of this literature are given by Jennings (1910, 1917), Pearl (1917), 

 and Sturtevant (1918). 



