FERTILITY 197 



case by the average number of flies per pair that 

 hatch. But this is not a measure of the number of 

 eggs laid or of those that are fertiUzed. 



Whether inbreeding where separate sexes exist is sim- 

 ilar to self-fertihzation in hermaphroditic forms is not 

 known. Darwin gives results of self-fertilization in Ijw- 

 moea purpurea for ten generations. The effects vary so 

 much in successive generations that it is not possible 

 to state whether or not the plant has become less 

 fertile. His evidence shows, however, that the cross- 

 fertilized plants in each of the same ten generations 

 are more vigorous than the self-fertilized plants, but 

 this does not prove that the latter deteriorated. 



The problem has been studied in other ways. Some 

 animals and plants propagate extensively by partheno- 

 genesis ; others by means of simple division. 



Whitney and A. F. Shull kept parthenogenetic strains 

 of Hydatina senta for many generations. Whitney 

 carried a strain of this sort through 500 generations. 

 Towards the end the individuals became weak, the 

 reproductive power was greatly diminished, and finally 

 the strain died out. No attempt was made to breed 

 from the more fertile individuals, although to some 

 extent this probably occurred at times. If we admit 

 that weakened individuals appear sometimes in these 

 lines and their weakness is inherited, then each time 

 such an individual happened. to be picked out a step 

 downw^ard would be taken ; when the more fertile 

 individuals chanced to be selected, the strain w^ould be 

 temporarily held at that level. But on the w^hole 

 the process would be downw^ards if such downward 

 changes are more likely to occur than upward ones. 



