A PHYSIOLOGICAL CHARACTER. 89 



time mean for the minus strain was higher four times and lower 

 twice. Further detailed analysis fails to show any relation between 

 mean reproductive index and mean reaction-time. 



The curves for Line 740 indicate clearly a marked general effect 

 of environmental conditions upon the reactiveness of these animals. 

 There are irregularities in the curve, but in a general way the reaction- 

 time curves for the two strains follow each other remarkably well. 



It is possible that a mutation occurred in the minus strain of 

 Line 740 in the forty-third generation of selection. A brood of 8 

 young on September 28, 1913, had the remarkably low reaction- 

 time mean of 159 seconds. These were day-old young, however, and 

 day-old young are sometimes abnormally reactive. Further, 4 of 

 these 8 young were negative in their reactions, an unusual result, 

 indicating unusual experimental conditions (see page 15). However, 

 on the following day a brood of 8 from another mother of this strain 

 had a mean of only 280 seconds. 



Examination of the other selection data for the same days 

 showed that the broods of the strains of D. pulex from which selections 

 were made were unusually reactive. Fairly extensive tests of addi- 

 tional broods of 740 minus and other strains of S. exspinosus extend- 

 ing over several generations failed to show any consistently greater 

 reactiveness in 740 minus, though certain descendants of the very 

 reactive individuals of the minus strain of Line 740 produced broods 

 some, but a minority, of which were very reactive. 



The results of these tests and the fact of the unusual reactive- 

 ness of many of the broods of other strains tested during this period 

 led to the conclusion that a mutation had probably not occurred and 

 that the unusual reactiveness of the minus strain of Line 740 at that 

 time had probably been caused by unusual experimental conditions. 



If a mutation did occur it should have remained apparent (in 

 the absence of segregation) at least for a considerable period, in 

 spite of any effect of selection. But after the second generation the 

 reaction-means for 740 minus were normal, running usually above 

 700 seconds. 



LINE 757. 

 PRESENTATION OF DATA. 



The data for Line 757 will be examined with great care, for with 

 this line the result is different from that with most of the other lines 

 used in these selection experiments, and it is desired to be conserva- 

 tive regarding any conclusions that may be reached regarding the 

 effect of selection within this line. 



The original mother of Line 757 was obtained October 19, 1912, 

 from Pond I, the temporary surface-water pond. Selection was 

 begun with her first laboratory brood of young and continued for 181 

 generations, covering a period of 43^ years. 



