EFFECT OF TEMPERATURE ON CROSSING OVER 197 
more frequent in certain regions than in others, and this is true 
not only when the regions compared are in different chromo- 
somes, but when they are in different portions of the same 
chromosome. For instance, for the black-purple-vestigial region 
of chromosome II we get a coincidence of 66 per cent,i while 
for distances slightly longer in chromosome I-scute to cut or 
cut to garnet — the values are 40 per cent and 22 per cent, respec- 
tively. The black-purple-curved region in chromosome II 
gives a coincidence value of 96 per cent (Plough, '17, table 8), 
while a similar length in chromosome I — echinus-cut-vermilion — 
gives 26 per cent. Even more striking, however, is the fact that 
the region at the upper end of the third chromosome on the 
map — sepia-Dichete-spineless, a distance of 28 units — gives a 
coincidence value of 122 per cent, ^''et the longer lower region — 
spineless-sooty-rough (33 units) gives only 55 per cent coinci- 
dence. This indicates that double crossing over is at least as 
frequent as though there were no interference at the sepia end 
of chromosome II, but interference is almost as high at the rough 
end as in chromosome I. A difference of the same order is 
apparent in chromosomes I and II. 
It will now be obvious that those sections of the chromosomes 
mapped in figure 3 which show a relatively high coincidence per 
unit of distance are the same ones which show a change in the 
amount of crossing over as a result of high temperature and of 
the age of the female. In no case where the coincidence value 
for a continuous region of 30 units or less is below about 60 per 
cent do we find an increase in crossing over due to high temper- 
ature, or, with the possible exceptions noted in the second chromo- 
some, a change due to age. The chromosomal regions which are 
'sensitive' to environmental effects all show a minimum of influ- 
ence of one crossover on another simultaneous crossover in the 
same region. 
1 Bridges and Morgan, '19, table 42 — not 61 per cent, as they give it. 
