168 SELECTION IN CLADOCERA. 



the pure-line hypothesis, but there is no evidence for the occurrence 

 in Cladocera of such unusual chromatic behavior. 



36. The divergence in reaction-time may be explained as due 

 to larger mutations. But the 'data do not show points, with one 

 possible exception, at which such mutations may presumably have 

 occurred, and, assuming a minimum of 3 mutations, each mutation 

 must have produced its maximum effect long after its occurrence, a 

 fact not consistent with the current conception of a mutation in such 

 material. Further, the mutations must have occurred in both 

 strains, and since three and probably more mutations are required, 

 this would call for a high rate of mutation in this line. 



37. The result may be explained as due to a gradual modifica- 

 tion, many small changes (mutations?), in the genetic factor or factors 

 involved. 





