Inbreeding 195 
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 
