674 
GENETICS: METZ AND BRIDGES 
behavior is very different; 'glazed' females by 'rugose' males have entirely 
failed to give offspring, and the reciprocal cross, although easily ob- 
tained, gives sterile hybrids (i.e., sterile females — these being the only 
hybrid individuals) . In other words, it has been impossible thus far to 
get an F2 generation from the cross, no matter how the mating was made. 
No accurate record has been kept of the earlier experiments with the 
two forms, but at least a score or more of matings were made at different 
intervals during six or eight generations. Among the more recent at- 
tempts, in which a record was kept of all Fi matings, the following may 
be cited : One experiment began with about a dozen reciprocal matings 
between glazed and rugose, using several flies in each bottle. At least 
three of these, in which glazed males and rugose females were used, gave 
many offspring. Fourteen large, vigorous females were selected from 
the latter and mated singly to several males — eight to rugose males, six 
to normal males. They were all given the best possible cultural condi- 
tions and treated exactly like other mutants of various kinds that were 
mated at the same time. Practically all of the latter gave abundant 
progeny, but not a single offspring appeared in any of the fourteen bottles 
containing glazed-rugose hybrids, though the flies lived for a long time 
(much longer than the ordinary length of one generation). Another 
experiment was made in a different way. This time females hetero- 
zygous for glazed were mated to rugose males, and females heterozygous 
for rugose were mated to glazed males. These gave daughters half of 
which were heterozygous for either rugose or glazed, and half of which 
were heterozygous for both rugose and glazed. Forty-five females from 
such cultures were mated singly to various males (mostly normal). 
Twenty of the females were from the first mating, twenty-five from the 
second. Out of the twenty females nine gave abundant offspring and 
eleven gave none; out of the twenty-five females fourteen gave offspring, 
eleven gave none; or, out of the total of forty-five, twenty- three gave 
offspring and twenty-two did not. In three cases the female died before 
the records were made, so the numbers are subject to that much error, 
but in all of the others the flies lived to twice or three times the age at 
which offspring are olrdinarily produced. Of the twenty-three females 
that did give progeny, every one proved to be heterozygous for only one 
of the characters concerned — i.e., those from the first mating were hetero- 
zygous for rugose but not glazed; those from the second mating were 
heterozygous for glazed but not rugose — leaving no doubt that the sterile 
females were those carrying both glazed and rugose. 
It is difficult to avoid the conclusion that we are dealing here with a 
complete, or at least a high degree of incompatibility. That it is spe- 
