GENES—STURTEVANT 
each species are located, the striking 
result is that those that are associated 
in any one element in one species lie 
also in the same element in every 
other species where they can be iden- 
tified. There is one exception to this 
rule shown in the diagram—in D. 
ananassae a part of element A is asso- 
ciated with element F—a relation 
that is known both from direct micro- 
scopic examination of the chromo- 
somes and from a comparison of the 
homologous genes, so that the excep- 
tion does not in principle upset the 
parallelism. There are also one or 
two other probable exceptions, con- 
cerning species that are as yet little 
studied and therefore are not well 
understood. 
This rule of the integrity of the ele- 
ments serves as a strong confirmation 
of the identification of the genes; for, 
if one were making many mistakes in 
identifying them, no such consistent 
result would be possible. In this 
connection it should be pointed out 
that there are many genes that cannot 
be utilized in such comparisons, be- 
cause the characters associated with 
them are not sufficiently specific. 
There are, for example, several differ- 
ent pinkish eye colors in most species: 
these are not distinguishable (with 
methods now available) within a 
species without the test of crossing, 
and they are therefore of little use in 
interspecific comparisons. Their pres- 
ence is not contrary to the scheme, 
but they cannot be used to support it. 
Comparisons of this same nature 
may be made between related species 
in several other groups, notably among 
the rodents and in several groups of 
seed plants. In no case can the com- 
parisons be pushed so far as in the 
genus Drosophila, since there is in these 
groups no thoroughly understood 
standard of comparison equivalent 
to D. melanogaster. The data, how- 
ever, are in agreement with those 
from Drosophila so far as they go; I 
think there can be no doubt that, in 
general, related species have essenti- 
ally the same complements of genes. 
297 
It must be recognized, however, 
that even in the best-known pair of 
species the total number of common 
genes indicated by this method lies 
only between 50 and 60, which is the 
number common to D. melanogaster 
and D. pseudoobscura—the uncertainty 
arising from the existence of more or 
less questionable comparisons. ‘This 
is only a small fraction of the number 
of different genes present in each spe- 
cies. It can be concluded, I think, 
that many more are in fact alike. If 
one considers the bristle patterns of 
the “wild type” it is clear that the 
two species are closely similar, and 
also that there are identical gene sub- 
stitutions which result in new patterns 
that are equally similar. This evi- 
dently means that the original patterns 
were alike, because their development 
was determined in the same way in 
each species. That is, not only the 
genes that give similar mutant types 
are the same, but essentially the whole 
set of genes controlling bristle develop- 
ment is the same. This argument, 
when applied consistently to the vari- 
ous characters for which correspond- 
ing mutant genes are known, leads to 
the conclusion that the whole animals 
are controlled by nearly identical 
systems of genes. 
Such a conclusion may seem to be 
merely a platitude. After all, these 
are very similar animals belonging to 
a single genus, and are much alike in 
most of their properties. It may be 
asked—was it worth while to spend 
so much effort in establishing their 
essential genetic similarity? Did any- 
one ever doubt it? The answer is that 
precisely this point has been seriously 
doubted, and so it has seemed desir- 
able to examine the situation carefully. 
II 
The basis for this doubt may perhaps 
be stated best by first going back a 
little in the history of genetics. The 
orthodox view has been that at each 
particular point, or locus, in each 
chromosome there is a “‘wild type” 
