160 SELECTION IN CLADOCERA ON THE BASIS OF 



environmental influences obviously were the determining factor). If, 

 assuming that environmental influences are responsible for the 

 fluctuations during parts of 1914 and 1915, one momentarily dis- 

 regards entirely the curves for the Line 757 strains during this period 

 of irregularities, the curves for the two strains following this period 

 are seen to be mere resumptions of the courses of the curves previous 

 to this period. 



Hence it is seen that if it is necessary to assume that environ- 

 mental factors exercised a determining influence, they may account 

 in full for the deviations in the curves from their general courses 

 during parts of 1914 and 1915, and there seems no necessity for, nor 

 ground for, the assumption of a large mutation in the minus strain 

 of Line 757 about August 1915; and, although this is the only point 

 in the curves at which a mutation of considerable moment may 

 seem to have occurred, the writer believes that another interpreta- 

 tion more logically accounts for the facts of the case. 



It therefore appears that the point of occurrence of any really 

 large mutation can not be satisfactorily located. Since the divergence 

 in reactiveness between the two strains of Line 757 became very 

 large and mutations of considerable moment can not be located, 

 larger mutations do not seem to account for the result in Line 757. 

 When one considers the further fact that a minimum of three larger 

 mutations would be required to explain the case on this basis, and 

 that, by analogy with the Drosophila case, this would call for an 

 extremely high rate of mutation, the plausibility of this explanation 

 still further decreases. 



The data for Line 757 readily permit of the interpretation of 

 the result as having been due to imperceptibly 1 small changes 2 

 resulting, through selection, in cumulative effects in definite direc- 

 tions. The result as shown in the curves (figures 18s and 19) readily 

 lends itself to this explanation, assuming that the writer's inter- 

 pretation is correct and that the irregular portions of the curves 

 (during parts of 1914 and 1915) are really due to disturbing en- 



1 At any rate, their occurrence was unperceived. Reaction-time in this study is read off in 

 seconds. The character studied, reactiveness to light, would seem ideal for detecting minute 

 genetic changes in that the smallest changes in reactiveness to light should presumably be de- 

 tected. But the profound influence of environment upon reactiveness proved extremely dis- 

 turbing and only by averaging large numbers of individual reaction-times could modifications 

 in reactiveness be detected. This was a distinct disappointment. In this study, because of 

 these fluctuations in reaction-time, every case in which there seems any reasonable doubt as to 

 an effect of selection is refused credit in conclusions favoring an influence of selection, but in 

 no case are the full experimental results withheld from the reader. 



2 These changes, one believes, arose quite independently of selection, but were utilized 

 through selection in building up the differences in reactiveness between the plus and minus 

 strains. Selection does not cause genetic change. It merely seizes upon modifications of the 

 character used in selection as they occur, and in the case of plural genetic changes may build 

 up differences between selected strains. However, retention (through selection) of an heredi- 

 tarily changed character, in so far as it preserves the material which may undergo further genetic 

 variation, makes possible a further change in the same direction in the same character. Selection, 

 although not a cause of variation, thus enables a deviation to be built up in the character utilized 

 in selection if genetic changes occur in the direction sought. 



