162 SELECTION IN CLADOCERA ON THE BASIS OF 



The explanation of the results of selection in this material as 

 having been due to many small genetic changes is suggested with 

 extreme caution. There is the objection that such an explanation 

 runs counter to the most acceptable explanation for most cases of 

 effective selection within the pure line and that many cases for 

 which this explanation has been invoked have later yielded to 

 analysis on the basis of demonstrable mutation or genetic segrega- 

 tion. But the objections to explanation of the present case as due 

 to segregation or larger mutations seem even more weighty (unless 

 one assumes a novel and quite unknown type of segregation in this 

 material), and it is believed that gradual modification of the factor or 

 factors involved in the present case must receive serious consideration. 



Since changes in a character seem to occur with such frequency 

 in some clones of Cladocera, material in which because of uniparental 

 inheritance and lack of reduction frequent genetic changes would 

 seem not to be expected, one wonders if this type of organism is 

 unique among metazoa and if such genetic changes may not likewise 

 be found to occur elsewhere. In other words, may not selection be 

 found equally successful in some other material and may not the 

 case for selection be more hopeful than is generally supposed? In 

 any material in which improvement or change in a character is 

 sought, genetic variation may (or may not) be encountered. If 

 genetic variation occurs in the character studied and in the direction 

 sought, selection (though not a cause of the variations) may be a 

 means of utilizing the -variations in accomplishing the end sought. 



The pure-line concept, set forth by Jordan and elaborated by 

 Johannsen and his followers, has been extended until it has been 

 shown to have wide application. Whether it may become universal 

 in its application remains to be seen. The writer believes that such 

 cases as the present are crucial in their relation to the universal 

 validity of the theory, and that if they are not (or until they are) 

 brought into line this concept can not attain the dignity of a law. 

 It is for this reason that the present case has been presented in so 

 much detail. 



Whatever explanation one finds acceptable, the facts are that 

 the effects of selection within Line 757 are due both to an increased 

 reactiveness in the plus strain of Line 757 and a decreased reactive- 

 ness in the minus strain of the same line (both relative to the corre- 

 sponding strains of the other lines of Simocephalus) , the two modifica- 

 tions working together to produce a large difference in reactiveness 

 between the two strains of Line 757; and that this difference was 

 permanent or at least persisted for 112 generations after selection 

 was discontinued. 



