168 "THE CHROMOSOMES 
there is no explanation as to why the factors in a 
group remain together in sections as explained on 
page 66. If it is supposed that the chromosomes 
break only once or twice, and that linkage represents 
the holding together of the pieces, then one is forced 
to assume that the breaking up is the same in both 
members of a pair, yet entirely inconstant in different 
cells; for otherwise the reunion of the fragments 
would lead to duplication or loss of whole sections 
of the chromosomes, and all order would soon be lost. 
A large amount of data relating to sex linked char- 
acters has shown that the sex chromosomes must 
remain intact as often as they break apart, and even 
when they break apart this takes place, as a rule, at 
only one place. 
OTHER THEORIES OF CROSSING OVER 
The ‘reduplication” theory of Bateson and 
Punnett has been treated in another section (see 
page 74) and the discussion need not be repeated 
here. 
It has been claimed by Goldschmidt that, even 
though the groups of factors are admitted to be 
carried in the chromosomes and to undergo some 
sort of interchange before reduction, it is not neces- 
sary to assume that this interchange is of the nature 
of a crossing over of the threads. He has postulated 
that during resting stages the genes are set free 
from the forces that hold them in their places in the 
chromosomes, and that when they reassume their 
places before division the two allelomorphs may 
