383 EXPERIMENTAL STUDY OF INHERITANCE 



result that " inbreeding probably reduces very slightly the 

 productiveness of Drosophila, but the productiveness may be 

 fully maintained under constant inbreeding (brother and sister) 

 if selection be made from the more productive families.'* 



Castle (1911) also reports that a polydactylous race of guinea- 

 pigs all descended from one individual remained exceedingly 

 vigorous for ten years and then showed no hint of diminishing 

 fertility. 



While it seems certain that prolonged and close inbreeding 

 may afford opportunity for an inherent taint to show itself, to 

 spread, and to accumulate, it 'is not the consanguinity that is 

 to blame for the taint. The same consequences would probably 

 result if matings took place among unrelated organisms with the 

 same kind of taint. 



In regard to cousin marriages in mankind the family history 

 should be carefully scrutinised. The likelihood of unhealthy 

 offspring will be very great if there are the same hereditary 

 taints in the lineage of both parents. If there is a well-defined 

 family predisposition to certain diseases, the fact that the 

 cousins are somatically healthy does not warrant their be- 

 coming parents. If two somatically healthy cousins belonging 

 to a tainted family have what is called a single or simplex 

 dose of the taint, the probability is that on the average 

 one quarter of the children would be unhealthy. On the 

 other hand, if the family history is good on both sides, 

 there is no biological reason why two healthy cousins, who 

 fall in love with one another, should not marry and have 

 children. 



Some variations are from the first so stable that their per- 

 sistence is certain without any precautions of inbreeding. But, 

 in other cases, it appears to be the experience of breeders that 

 a period of inbreeding, with elimination of any " weeds " that 

 may crop up, serves to fix characters, developing " prepotency " 

 in regard to the desired qualities. Crossing may then be resorted 



