a 
118 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 
produce, by D. simulans males, only regular daughters and exceptional 
sons. The exceptional daughters and regular sons die. In every case 
it has been possible to determine, by means of sex-linked genes, that 
the female hybrids carry two X chromosomes, the males only one X, 
as is the case in both parent species. 
The hybrid females from the two crosses first mentioned must have 
the same chromosomal constitution; yet in one case they live, while 
in the other they usually die. This can only mean that the result is 
due to an interaction between the chromosome complex and the egg 
cytoplasm in which it finds itself, unless selective fertilization occurs. 
It seems probable that in this case, as in other instances in which the 
cytoplasm plays a part in the result, the nature of the cytoplasm is 
itself determined by the chromosome complex of the female that pro- 
duced the egg. 
These data indicate that the two species differ in a number of genes. 
Not only are they slightly different in appearance in a number of 
ways, but the same end-results (viability, fertility, bristle-number, 
abdominal banding, wing-shape, venation) must be brought about by 
somewhat different sets of genes in the two species, since when both 
sets are present in the same animal the end result is different from 
that which either one alone would produce. 
As stated above (p. 14) mutations have been found in D. simulans 
that appear to be identical with previously known ones in D. melano- 
gaster, and in five cases crosses have shown that the mutant genes in 
the two species are allelomorphic. Four of these are sex-linked, 
and have the same linear sequence in the two species, though they 
do not show quite the same frequencies of crossing-over. This fur- 
nishes a demonstration that the germ-plasms of two distinct species 
contain some identical genes that are subject to identical mutations. 
