6 SELECTION IN CLADOCERA ON THE BASIS OF 



The rapidly breeding, easily propagated, parthenogenetic organ- 

 ism possessing a readily measurable physiological character afforded 

 just the material desired. 



The experimental object was to attempt to isolate by selection, 

 through a number of generations, a strain more responsive to light 

 than a second strain from the same pure line 1 selected for its reduced 

 responsiveness to light. 2 



SOURCE OF MATERIAL. 



The Cladocera material to be considered in this paper was 

 obtained from ponds near Cold Spring Harbor. Pond I is a surface- 

 water pond in an open lot at an altitude of about 160 feet above tide- 

 water. This is a temporary pond, some 60 by 40 feet in dimensions 

 when moderately filled, and contains water only from early winter 

 to May or somewhat later in the summer. Pond II was 3 a permanent 

 spring-fed pond in a shaded situation in the woods on a northeast 

 slope. It was about one-fourth mile distant from Pond I, at an 

 altitude of about 60 feet, was approximately 18 by 10 feet, and had 

 a depth of only 10 inches. Pond III is another surface-water pond 

 on the upland, three-eighths of a mile from Pond I. It overflows 

 occasionally after excessive rains and frequently becomes dried up in 

 summer. 4 At its maximum it is approximately 65 by 40 feet, but 

 it ordinarily contains less than a fourth of this area and does not 

 exceed 2 feet in depth at its deepest portion. This pond is near 

 and receives the surface drainage from a barnyard and a pasture lot. 

 Hence it is rich in organic solution constituents. Pond IV is also a 

 shallow surface-water pond. It is on the upland, an eighth of a mile 

 from Pond I, ordinarily has an area of perhaps half an acre, and has 

 become dry only once in the past 7 years. This pond also receives the 

 surface drainage from a barnyard, but because of its larger drainage 

 basin it is not so rich in organic solution constituents as Pond III. 



The Daphnia pulex material used in the selection experiments 

 was obtained from Pond I (November 1911 and October 1912) and 

 from Pond II (November 1911). Only two lines (689, 751) from 

 Pond I were retained very long. The remainder of the lines of 

 D. pulex used in these experiments came from Pond II, the small 

 spring-fed pond in the woods. The D. longispina used in selection 

 were derived from stock from Ponds I and IV. Collections were 



1 These were not "pure lines" in the narrowed sense of Johannsen's definition, but were 

 clones in the sense adopted by Johannsen and his followers. The term "pure line" has not in 

 the general mind been restricted to Johannsen's usage, however. Nevertheless, to avoid loose- 

 ness of expression and possible misunderstanding, it might be wise to abandon this term to 

 Johannsen's limited meaning and for a general term, embracing the Johannsen pure line and the 

 clone, to adopt the term pure lineage or pure descent. 



*Two short notes relating to this series of experiments have been previously published 

 (Banta, 1913, 1919). The references there made to Simocephalus vetulus should read Simo- 

 cephalus exspinosus = vetulus. 



3 It has since been drained. 



4 It has become empty in the autumn three times in the last 7 years. 



