294. 
higher plants, and in recent years we 
have come to know a good deal about 
the genetics of some of the lower plants. 
Perhaps the most important general 
conclusion to be drawn from a com- 
parison of heredity in these forms is 
that, by and large, the same principles 
apply to all of them. There are dif- 
ferences, but these are not sufficient 
to obscure the essential similarity. 
For some comparative purposes, 
however, it is desirable to consider a 
series of rather closely related forms. 
The best understood group of related 
species occurs in Drosophila—a genus 
of small flies, 2 to 6 mm. in length, 
many of which are convenient labora- 
tory objects. The best known is Dro- 
sophila melanogaster. This is a small 
yellowish fly with bright red eyes, 
commonly found about fermenting 
fruit in most parts of the world. Ow- 
ing to its short life cycle of about 10 
days and to the fact that large num- 
bers of the flies can be reared with 
little space and expense, D. melanogaster 
has long been a favorite subject for 
genetical research. More is known 
about heredity in this species than in 
any other organism, and much of the 
modern theory of genetics is based on 
studies of it. 
There are about 500 known species 
of Drosophila; of these about 8 have 
been reasonably well studied geneti- 
cally, as many more are less well un- 
derstood, and there are scattered data 
on still others (8).2_ The species so far 
studied do not constitute a random 
sample of the genus, since several of 
the more distinct groups of species do 
not breed easily under usual laboratory 
conditions; nevertheless the available 
species do represent considerable diver- 
sities of type. 
There are a few species hybrids rec- 
ognized in Drosophila, and the study of 
these has yielded much information; 
in all cases, however, interfertile species 
are so closely similar as to be of little 
interest for present purposes. For the 
2? Numbers in parentheses indicate refer- 
ences at end of article. 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
other forms, here under consideration, 
the method of study has been to in- 
vestigate each species separately, and 
then to compare the information on 
the gene composition of a particular 
species with that available for the other 
species. This usually means a com- 
parison with melanogaster, since more 
genes are known in it than in all the 
other species combined. 
The methods used may be illustrated 
by a few examples. D. melanogaster, as 
stated, is commonly found in most 
parts of the world. Although there 
are minor differences between strains 
in this species, there is, nevertheless, 
essentially a single “‘wild type” form 
to be found everywhere and it is not 
possible to judge the geographical 
origin of a specimen or of a strain by 
an examination of its characteristics. 
This “wild type” form is the standard 
of reference for comparison of gene 
composition. Occasionally one finds, 
either in laboratory cultures or in wild 
populations, individuals with definitely 
distinct characteristics, and such a new 
feature can often be shown to be due 
to a change that has occurred in a 
single gene. Since the resulting new 
genes are relatively stable, it is possible 
to establish strains carrying them, and 
to produce any desired number of in- 
dividuals showing the distinct new 
characters. 
One such new, or “‘mutant,”’ gene 
results in flies possessing shortened, 
kinky bristles. Specimens with this 
character agree with the “‘wild type” 
in all but one of the thousands of 
genes they carry; the kinky bristles are 
the consequence of a single gene sub- 
stitution. It happens, however, that 
there are two different genes in the 
‘‘wild type,’ a change or mutation in 
either of which will produce kinky 
bristles; the two resulting mutant char- 
acters are not distinguishable by su- 
perficial examination of the flies. Each 
of these genes in the “‘wild type” un- 
dergoes mutation relatively frequently: 
it is, therefore, possible to obtain a 
considerable series of separately arisen 
strains with such bristles. Crosses be- 
