158 SELECTION IN CLADOCERA ON THE BASIS OF 



tion," although he states that in the much-mutating Drosophila the 

 bulk of the mutations occur "at or very near the maturation stage." 

 In parthenogenetic reproduction in Cladocera, with only one matura- 

 tion division (and that a non-reduction division) the probability of 

 mutation would seem further greatly reduced. Hence, so far as one 

 may judge, mutations in Cladocera should be expected to be much 

 less frequent than in Drosophila. In the latter, Muller and Alten- 

 burg (1919) state that mutation in a particular factor is infrequent. 

 From their data,^ mutation in a particular factor occurs about once 

 in 50,000 generations. Yet this is the much-mutating Drosophila. 

 Hence, if one may judge from the evidence from Drosophila, mutation 

 in parthenogenetic reproduction in Cladocera should be very rare. 



The selection experiment with Line 757 covered 181 generations. 

 If larger mutations (affecting reactiveness to light) occurred, there 

 must have been one (at least) in the plus strain and two (at least) 

 in 181 generations in the minus strain, a surprisingly high incidence. 

 It is possible that several genetic factors are concerned in the photo- 

 tropism of Simocephalus, though McEwan (1918) found that a single 

 mutant factor in '*tan" Drosophila profoundly modified its photo- 

 tropism. But even if several factors are concerned in the present 

 case, at the rate of one mutation in a particular factor in 50,000 

 generations (for which Muller and Altenburg give data for Droso- 

 phila), the rate of assumed mutation in Line 757 would seem most 

 excessive. If one disregards the possible, though improbable, muta- 

 tion in the plus strain of Line 740, and the possible, though unproven, 

 genetic modifications in Lines 689, 691, 711, 719, 762, 794, and 795, 

 of the Cladocera lines subjected to selection, Line 757 alone showed 

 mutation. If larger mutations are to account for the result with 

 Line 757, mutation must have been extremely frequent in this line 

 and rare in the other Cladocera lines studied. 



It seems, then, that there is difficulty in explaining so many 

 genetic changes as must have occurred in Line 757 during a relatively 

 short period as due to larger mutations, such as have been worked out 

 for Drosophila. 



If one seeks in the curves (figure 18b) for possible points of 

 occurrence of larger mutations, three points at which such seem 

 more nearly possible than elsewhere are seen in the curves, about 

 April 1913 for the minus strain, about January 1915 for the plus 

 strain, and about August 1915 for a second possible mutation in the 

 minus strain. But the data by broods do not indicate points at 

 which any mutation seems to have occurred. It is possible, of 

 course, that the rather widely fluctuating averages by broods might 

 tend to mask the occurrence of a mutation, but even so one would 



' Conklin criticizes Muller and Altenburg's statement regarding the frequency of mutation, 

 but even if this statement is imwarranted it will still readily be granted that mutation in a 

 particular factor is infrequent. 



