360 DAVID DAY WHITNEY 
ably proves it; but as only the descendants of one cross fertilized 
egg were studied and since there was a lack of all time data and 
a failure to record the number of parthenogenetic mothers used 
in each generation, this table can not be taken as very conclusive 
evidence. This tentative conclusion of Shull’s was not known 
until after nearly all the above results of this paper were ascer- 
tained. 
Castle and his collaborators in breeding experiments with Dro- 
sophila state that: 
A cross between two races, one inbred for thirty or more generations 
and of low productiveness, the other inbred for less than ten generations 
and of high productiveness, produced offspring like the latter in produc- 
tiveness but not superior to it. The same two races crossed after an 
additional year of inbreeding (about twenty generations) produced 
offspring superior to either pure race in productiveness. 
This seems to be in the final results a case parallel to that of 
Hydantina senta. When two races have been closely inbred, 
like brother and sister for many generations or even when two 
races have been bred parthenogenetically, which is the extreme 
of inbreeding, for many generations they show in cross-breeding 
a great increase in productiveness of offspring which is superior 
to that of either of the parent races. 
The effect of inbreeding among animals has been of consider- 
able interest and is of great practical importance; but even at the 
present time there is much diversity of opinion in regard to the 
matter. The relation of inbreeding to sterility has been observed 
in experiments upon mammals by Crampe, Bos, and Guaita; 
upon birds by Fabre-Domengue. They all found the relation to 
be a causal one, continuous inbreeding, as of brothers and sisters, 
resulting in a decrease of fertility, accompanied more or less by 
lack of vigor, diminution in size, partial or complete sterility, and 
pathological malformations. 
In inbreeding experiments upon the pomace-fly, Drosophila, 
Castle and his collaborators state that ‘‘inbreeding probably re- 
duces very slightly the productiveness of Drosophila.” 
Moenkhaus has recently completed inbreeding experiments 
upon Drosophila in which he has inbred brothers and sisters for 
