GENETICS: STURTEVANT, BRIDGES, AND MORGAN 169 
According to Castle, "that the arrangement of the genes within a Knkage 
system is strictly linear seems for a variety of reasons doubtful." The only 
one of these reasons specified is that "it is doubtful, for example, whether an 
elaborate organic molecule ever has a simple string-like form." The sugges- 
tion that chromosomes are organic molecules is probably not intended to be 
taken seriously. The further argument, that organic molecules probably 
never have a simple string-like form, is scarcely to the point, for chromosomes 
do have a thread-like form. The comparison between crossing over and 
organic substitution reactions seems forced, for these reasons, as well as on 
strictly chemical grounds. 
Castle presents two cases as "critical" ones showing that the arrangement 
cannot be linear; viz., the relations of bifid and of abnormal to yellow and 
white. The case of abnormal presents some complications,^ but bifid may be 
accepted as a crucial case. On the basis of the data summarized by Morgan 
and Bridges^ and used by Castle, the crossover values for yellow, white and 
bifid are: 
Yellow white 1.1 
White bifid , 5.3 
Yellow bifid 5.5 
Each of these values is based on several thousand flies, so that the probable 
errors arising from random sampling are small. Castle points out that no 
one of the three values is either the sum or the difference of the other two, 
and therefore concludes that the placing of the three loci in a line cannot 
represent their relations to each other. But these data come from several 
sources; and it has frequently been pointed out that crossover values are 
subject to variation, due to genetic factors,^ to environmental causes,^ or to 
differential viability.^ In such a case as the present one, then, strictly coixi- 
parable data can be obtained only from experiments in which all three loci 
are followed at the same time.'^ Such crucial data for yellow, white, and 
bifid had been given by Morgan and Bridges^ and by Muller,^ in papers which 
were known to Castle. These experiments may be summarized as follows: 
NON-CROSS- 
OVERS 
YELLOW- 
SEPARATES 
FROM OTHER 
TWO 
WHITE 
SEPARATES 
BIFID 
SEPARATES 
TOTAL 
Morgan and Bridges 
MuUer 
487 
673 
3 
12 
0 
0 
16 
. 27 
506 
712 
Total 
Percentage 
1160 
15 
1.2 
0 
0.0 
43 
3.5 
1218 
Here we have crossover values that are strictly comparable with each other, 
and they show: 
