A PHYSIOLOGICAL CHARACTER. 73 



GENERAL INTRODUCTION FOR SIMOCEPHALUS 

 EXSPINOSUS LINES. 



S. exspinosus in some regards seemed an unfavorable species for 

 selection on the basis of its reaction to light. During the tests indi- 

 viduals frequently attached to the surface film or to the sides of 

 the experimental tank and rested there during the remainder of the 

 test (see also page 25). Further, and to an even greater degree, the S. 

 exspinosus young were unsatisfactory in that they so generally settled 

 to the bottom of the tank and appeared non-reactive to light. In 

 most broods there were several individuals which failed to reach 

 either end of the tank during the 15 minutes of the experiment, while 

 in some broods (particularly during the earlier course of the experi- 

 ments) there was no response on the part of any individual of the 

 brood. The reactiveness to light during the early course of the ex- 

 periments was so slight that in many cases there were really no 

 grounds for making a selection and there seemed little hope of a 

 sufficient reactiveness to light to afford a basis for conducting an 

 experiment in selection on this character. 



Several of the earlier S. exspinosus lines were discarded as un- 

 profitable after a few generations of selection. Lines 740 and 757, 

 while not promising at the start, were continued and subjected to 

 selection for a time to see if it then seemed advisable to continue the 

 selections. Even comparatively slightly reactive material might 

 conceivably afford a basis for selection if there were enough reactive 

 individuals to make a selection possible in a considerable percentage 

 of cases. In addition to broods (particularly the earlier broods of 

 these two lines) which showed absolutely no reaction to the light, 

 there were other broods in which the only selection possible (in the 

 plus strain) was from among individuals which had moved very 

 slightly toward the light. In the minus strains the only choice in a 

 vast majority of cases was an individual which showed no reaction to 

 light. There were numbers of these in nearly every brood. Negatively 

 reacting individuals were so rare with this species that it was seldom 

 possible to make selections in the minus strains on the ground of a 

 negative reaction to light 1 . But after a few generations it was con- 

 sidered that the reactiveness of the strains of Lines 740 and 757 

 seemed sufficient to justify the continuation of these lines in the hope 

 that there was sufficient reactiveness to test out the possibility of 

 selection. The results justify the conclusion then reached, that after 

 all there was sufficient basis for an experiment in selection. 



It is worthy of emphasis that the selections in the minus strains 

 of lines of Simocephalus were rarely on the ground of a negative 



1 However, as noted elsewhere (page 16), it is questionable if there is any real significance to 

 be assigned to the negative reactions of the few individuals which went to the negative end of 

 the experimental tank. 



