THE GERM-PLASM THEORY 307 
smaller elements. These differently sized dyads are 
considered by Carothers as “distinct physiological 
individuals, representing respectively the paternal 
and maternal contribution to the formation of some 
character or characters; and, as each can be iden- 
tified, they furnish an excellent means of tracing the 
process of segregation and recombination ”’ (p. 499). 
It was at first assumed that each of the pairs of 
chromosomes which unite in synapsis was responsible 
for a single adult 
character, but a—p 
the number of qd &? CO 
Mendelian char- Cc hy Po) oF 
acters is known 
. Fig. 83.— Arphia simplex. Chromosomes ot 
to be greater m first spermatocyte. a= accessory chremo- 
certain cases than some. 6 = unequal dyad. (From Carothers, 
1913.) 
the number of 
chromosomes. Fortunately, it has been found that 
the characters, instead of undergoing independent as- 
sortment, may become linked so that certain of them 
almost always occur together in the offspring. The 
relation of these facts to the constitution of the 
chromosomes may best be illustrated by reference to 
the studies of Morgan and his students on the fruit- 
fly, Drosophila. Over one hundred mutants of this 
species have been discovered by these investigators. 
So far as studied, the characters of these flies seem to 
form three groups. ‘‘The characters in the first group 
show sex-linked inheritance. They follow the sex- 
chromosomes. The second group is less extensive. 
Since the characters in this group are linked to each 
