A PHYSIOLOGICAL CHARACTER. 159 



expect a mutation of considerable import to appear evident very 

 quickly in the brood averages and one would seem justified in expect- 

 ing a mutation of considerable import to produce a marked and 

 immediate change in the two-month curve for the strain affected. 

 But such marked and immediate shifts are not found in the curves 

 (figure 18B), except possibly in the Line 757 minus curve following 

 August 1915, and even there a mutation seems not to have occurred 

 (see below). 



Since under the influence of the first two larger mutations pro- 

 visionally assumed to have occurred (but for which no satisfactory 

 locations can be found) the curves do not show great divergence, in 

 comparison with their later divergence, the wide divergence between 

 the curves for the two strains during the latter part of the experiment 

 seems to call for a mutation of large moment if the result is to be 

 explained by larger mutations. The divergence in the curves after 

 the third point (August 1915), at which a larger mutation may 

 possibly have occurred, seems somewhat pronounced, but does not 

 appear abrupt enough to have been due to the influence of a single 

 genetic change, unless the influence of this mutation was masked for 

 a time by a pronounced influence of environmental factors. It is, 

 of course, possible that this may have been the case. 



If, however, it is necessary to concede a marked influence of 

 environmental factors, a more logical explanation of the curve for 

 the minus strain would seem to lie in the interpretation that the irreg- 

 ularities in the curve for the minus strain during the period June 

 1914-August 1915 were due entirely to marked local environmental 

 influences. There seems no logical objection to this explanation. 



Evidence that this is the correct interpretation is seen in the wide 

 fluctuations, parallel for the most part to those for the minus strain, 

 in the Line 757 plus strain for the same period. 1 Further evidence 

 that this may be the correct interpretation is seen in the fact that 

 fluctuations similar to those found in the strains of Line 757 during 

 the period under discussion are seen during certain periods in other 

 strains, e. g., in strains of Lines 695 and 740, in which genetic changes 

 obviously had not occurred and in which only local environmental 

 influences seem to account for the facts. 



During this time of great fluctuation in mean reaction-times 

 in "the two strains of Line 757, there were three relatively very low 

 points in the Line 757 minus curve. Except for these low points, 

 the curve for the minus strain is seen to have held to a fairly 

 consistent course throughout the entire experiment (except for one 

 two-month period, December 1916-January 1917, in which local 



1 Except for the June-July 1914 period the fluctuations in the curves for the two strains of Line 

 757 are remarkably similar, though it is obvious that the Line 757 minus strain was somewhat 

 more influenced by environmental factors than the plus strain (figure 18B). It is interesting to 

 note that this June-July 1914 period is the only period of the entire experiment in which the 

 minus strain of Line 757 appeared more reactive than the plus strain. 



