A PHYSIOLOGICAL CHARACTER. 161 



vironmental influences as the peculiar results for the December 1916- 

 January 1917 period clearly are. 



Except for the fluctuations in the curves for the period men- 

 tioned, the curves for the two strains of Line 757 diverge rather 

 gradually and somewhat uniformly. There are considerable irregu- 

 larities in the curve for the plus strain, both during the period when 

 irregularities occurred in the minus strain and elsewhere, but despite 

 irregularities the general appearance of this curve strongly suggests 

 that its true course is approximately a straight line. The general 

 course of the curve for the minus strain less strikingly appears to be 

 a straight line, but, aside from the irregularities already pointed out 

 and accounted for, this curve is as regular as most curves based upon 

 similar biological material. 



The courses of the curves at the end of selection are such as 

 to suggest that the divergence in reactiveness between the two 

 strains was still increasing when the experiment closed. 



Many workers would prefer to call these small genetic changes 

 mutations, but they are indistinguishable from continuous gradual 

 changes and must have occurred with a frequency quite unknown in 

 forms in which the points of occurrence of definite mutations have 

 been recognized. Jennings (1916, p. 526) applies the term mutation 

 to such frequent small changes in morphological characters in 

 protozoa, and Hegner (1919) calls such small structural changes 

 (arising in his Arcella) mutations. But the complicated and less 

 well-known conditions of the nuclear material in protozoa and the 

 direct continuity between the cytoplasm, which one may think of as 

 soma, of the parent and that of offspring lead one to feel that at 

 least there is no positive assurance that conditions in protozoa are 

 entirely comparable with metozoa. 



In parthenogenetic Cladocera, without the reduction matura- 

 tion division and apparently lacking in unequal distribution of 

 nuclear elements, such as may occur in division in protozoa (Hegner, 

 1919), frequent mutation would not seem to be expected, since such 

 frequent mutation as would be called for in the present case has not 

 been recognized in metazoa and the mutations which have been most 

 frequent have generally appeared to be associated with maturation. 

 So many mutations through a lapse of only 181 generations would 

 call for mutations at a truly phenomenal rate. However, the matter 

 of a name for the genetic change merely turns upon one's definition 

 of a mutation. The facts of the present case are in nowise altered, 

 whether the small changes in character are termed mutations or 

 given other designation. It is possible that modifications of genes 

 are frequently of a low order and consequently not generally recog- 

 nized; further, that although they are not to be found wherever 

 sought, once such genetic change is initiated, such variations may 

 be frequent in occurrence. 



