190 Experimental Zoology 
degree the abhorrence of defilement by contact with other castes 
or with certain animals. 
George Darwin, who examined the question, as far as the sta- 
tistics were known, found no clear evidence of any evil effects 
caused by close intermarriage, or at most the evil was very small 
in amount, and out of all proportion to the prejudice against 
consanguineous unions. One might easily offer a number of 
possible explanations of the basis for this feeling, but such specu- 
lations would have little scientific value in the present state of 
our knowledge. 
Experiments with the Pomace Fly 
The most extensive series of experiments on inbreeding that 
have been made in recent years are those of Castle and his pupils. 
They have used the fruit or pomace fly, Drosophila ampelophila, 
which can be easily kept in confinement, and will breed through- 
out the year in a warm place. The whole life-cycle may 
be completed in eleven or twelve days (egg three, larva three, 
pupa three days, and two days before the imago begins to lay 
eggs). 
Brothers and sisters were paired in each generation. A pair 
of pupz of the same parentage was put into each jar. If the 
flies proved to be of different sexes, they were left together; if of 
the same sex, they were rearranged. The larve left the fruit 
on which they had fed and pupated on the top or sides of the 
glass. They were collected and counted, and the productive- 
ness of the pair calculated on this basis. Sometimes no eggs 
were produced, which was at times due to the sterility of the 
male, at other times of the female, as shown by pairing them with 
other individuals. 
The longest, or A-series, was carried through 59 generations 
extending over three years. We can appreciate the extent of the 
series if we compare it with the time required to produce the same 
number of generations of mankind. If we allow three genera- 
tions to a century, it would take 20 centuries to give 60 genera- 
tions. In other words, a corresponding experiment on man would 
