68 LINKAGE 
crossing over represents precisely the distance between 
the factors, for it may be that crossing over is more 
likely to take place in one region of the chromosome 
than in another. In that case the distances between 
factors in this region calculated from the amount of 
crossing over between them, would be relatively 
greater than the actual distance. It is supposed, 
however, that at least the order of the factors in the 
diagram represents their real order. Sturtevant 
has found definite factors which alter the amount 
of crossing over in the chromosomes, and these factors 
actually do affect the amount of crossing over differ- 
ently in the different regions. A map of the chromo- 
somes based upon the per cent. of crossing over when 
these factors are present would show different rela- 
tive distances between the loci than those calculated 
from the normal linkage values. It is to be noted, 
however, that even in these diagrams, the order of 
the factors remains unchanged. One of the factors 
lies in the second chromosome and lowers the amount 
of crossing over in certain regions of this chromosome; 
the other lies in the third and apparently affects 
only this chromosome, and chiefly the end of this 
chromosome in which it itself is located. Bridges has 
found that the percentage of crossing over in the sec- 
ond chromosome is also lowered with increase in the 
age of the female, and Plough has found that temper- 
ature as well may affect the amount of crossing over. 
This variation in crossing over is in no way preju- 
dicial to the conception of crossing over above out- 
lined. Variation in the amount of crossing over has 
