SELECTION IN CLADOCERA ON THE BASIS OF A 

 PHYSIOLOGICAL CHARACTER. 



GENERAL STATEMENT. 



For more than 8 years (January 1920) the writer has been rearing 

 parthenogenetic "pure lines" of Cladocera. The original object of 

 rearing this material was the conducting of experiments on selection 

 within the pure line. These experiments were completed in May 

 1917. Data bearing on other problems have also been secured from 

 the handling of this material. The selection experiments only will 

 be treated in the present paper. 



The writer undertook the selection experiments in order to get 

 additional data on the effects of selection within the pure line. The 

 Cladocera material was chosen for several reasons: 



1. It reproduces rapidly. 



2. Under favorable conditions it is readily handled in the 

 laboratory. 



3. It reproduces parthenogenetically with (under carefully con- 

 trolled conditions) no possible question as to the occurrence of sexual 

 reproduction. 



4. In the maturation of the parthenogenetic eggs of Cladocera 

 there is a single division without reduction (Weismann, 1886; Kiihn, 

 1908). Hence with this material there are presumably no compli- 

 cations of segregation during maturation or of fertilization as with 

 sexually reproducing forms. 



5. No selection experiments had been made with a purely 

 physiological character as the basis for selection. Aside from 

 the fact that a physiological character had probably not been used 

 previously as a basis for studies on the effect of selection, a purely 

 physiological character seemed desirable to use for two additional 

 reasons: (a) Physiological differences are frequently readily measur- 

 able and measurable in a precise way. Numerical series of measure- 

 ments provide very usable series of data free from the errors of 

 estimation and personal equation applying to series not directly 

 numerical. The light reactions of Cladocera afford a character per- 

 haps as definitely measurable as any readily handled physiological 

 characteristic. (6) In both the ontogenetic and phylogenetic history 

 of organisms physiological modifications may occur without observed 

 morphological changes, and in the modification of organisms physio- 

 logical modifications may readily precede distinguishable or measur- 

 able morphological changes. This latter consideration (b) had the 

 greatest weight in determining the use of a physiological character 

 as the basis for these experiments. 



