Inbreeding 1 95 



Castle's general conclusion is that "inbreeding probably re- 

 duces very slightly the productiveness of Drosophila, but the 

 productiveness may be fully maintained under constant in- 

 breeding (brother and sister) if selection is made from the more 

 productive families. There are also indications that cross- 

 breeding increases the productiveness of closely inbred families- 



Behavior of Germ-cells in Inbreeding 



Whether the decrease in fertility observed in some cases of 

 inbreeding is due to the failure of the spermatozoa to enter the 

 eggs, or to the failure of the fertilized eggs to develop, or to both 

 factors, cannot be stated, but there are a few observations that 

 show, indirectly, that the germ-cells themselves may be affected. 

 These cases unfortunately apply only to hermaphroditic species. 



It has been known to botanists for a long time that in certain 

 flowers pollen will not fertilize the ovary of the same plant. In 

 other cases where self-fertilization will occur, the pollen of other 

 plants is prepotent over that of the same plant. It seems not 

 improbable that the failure of the pollen to fertilize its own ovary 

 is due to the failure of the pollen tube to grow down into its own 

 stigma or style sufficiently far to reach the ovules. The pre- 

 potency of foreign pollen would be due on this view to more 

 rapid growth than that of the pollen of the same plant. 



Only one case of this sort is known in animals. Castle dis- 

 covered that the spermatozoa of the hermaphrodite ascidian, 

 Ciona intestinalis, fail, as a rule, to fertilize the eggs of the same 

 individual, although they will fertilize the eggs of any other indi- 

 vidual. I have carried out a series of experiments on this and 

 other species in the hope of discovering to what this action is due. 

 The results are complicated and difficult to interpret. 



In the first place the sperm of an individual (Ciona intestina- 

 lis) does not fertilize equally well the eggs of all other individuals. 

 In some cases 100 per cent of the eggs are fertilized ; in others 

 25 per cent or less ; but I have seen no clear case where the sperm 

 in good condition is as sterile with eggs of any other individual 

 as with its "own" eggs. The failure to fertilize properly is due 



