156 SELECTION IN CLADOCERA ON THE BASIS OF 



If the variations are considered direct genetic changes (or 

 general physiological changes having a genetic basis), thej' may be 

 thought of as having been due to segregation, 1 to larger mutations, 

 or to many small genetic changes. 



It would seem that the effect of selection here obtained can not 

 be explained as due to recombination or segregation unless segrega- 

 tion occurred rather generally in Line 757 under conditions in which 

 it is not known and is theoretically not supposed to occur. It is 

 conceivable, however, that segregation may occur on rare occasions 

 in Cladocera in the germ-tract previous to maturation, in the one 

 maturation division (even though that is a non-reduction division), 

 or in cleavage or later somatic divisions. But this explanation for 

 the result with Line 757 encounters certain difficulties: (1) Such 

 factorial recombination, if large quantitatively, should at once pro- 

 duce an obvious effect. (2) If, on the other hand, the factorial re- 

 combination produces only a slight effect, the result obtained with 

 Si?nocephalus Line 757 is explicable only as the result of several 

 of \hese exceptional factorial changes. (3) The effect is in both high 

 and low selection strains. 



Since with Line 757 the effect is extensive, is in both strains, and 

 appears gradually and not by a sudden increment (except possibly, 

 but doubtfully, at one point in the minus strain), this sort of explana- 

 tion would call for more than a single factorial recombination. 

 Probably many more genetic changes occurred, but at least three 

 such genetic recombinations are required to explain the case on this 

 basis. Such might possibly be located as follows: one in the minus 

 strain early in its laboratory history, one in the plus strain, possibly 

 about January 1915, and one in the minus strain about August 1915 

 (figure 18b). But the points at which these three or more hypothet- 

 ical recombinations occurred are not indicated in the data by broods 

 (tables 41 and 42) and can be doubtfully and only approximately 

 located (assuming 1 that they were masked for a time by environ- 

 mental conditions) by reference to the averages by two-month periods 

 (tables 43 and 44 and figure 18b). Yet presumably these, at least 

 the third one 2 (assuming a minimum of three genetic modifications), 

 must have been genetic changes of large moment, considering the 

 wide differences in reactiveness between the two strains during the 

 latter portion of the experiment. Since these factorial changes must 

 have come about under exceptional conditions, three or more such 

 occurrences affecting two strains of the same line within a limited 

 period seem unlikely of attainment. 



Note. — There is another possible suggestion concerning segregation in this material: 

 that notwithstanding parthenogenesis and the presumed lack of segregation in maturation 

 there may be a mechanism in Cladocera by which changes in genetic constitution may, 



1 Including the phenomena of crossing-over and non-disjunction. 



2 As shown on page 159, there is strong probability that no large genetic change occurred 

 at this point. 



