308 GERM-CELL CYCLE IN ANIMALS 
other, we say that they lie in a second chromosome. 
The characters of the third group have not as yet 
been so fully studied, except to show that they are 
linked. We place them in the third chromosome 
without any pretensions as to which of the pairs of 
chromosomes are numbered II and III. 
“The arrangement of these characters in groups 
is based on a general fact in regard to their behavior 
in heredity, viz., A member of any group shows linkage 
with all other members of that group, but shows inde- 
pendent assortment with any member of any other 
group.” If the factors which determine these groups 
of characters are situated in the chromosomes, as the 
hypothesis demands, we should expect each group 
to act as a unit in heredity. Occasionally, however, 
the characters of a group appear to act independently, 
and there must thus be an interchange of factors at 
the time of synapsis. As already stated (p. 254), an 
interchange of substances between chromosome pairs 
during synapsis is possible and even probable. Mor- 
gan explains the degree of crossing over of characters 
in the following way: The factors which determine 
the characters are arranged in the chromosomes in 
a linear series; those factors that are near together 
will have less chance of being separated than those 
that lie farther apart. The relative distances be- 
tween these factors can be judged by the frequency 
of interchange as determined by breeding experi- 
ments. It has thus been possible to locate certain 
factors in the chromosomes more or less accurately 
and to predict with some degree of certainty the re- 
