CHEO.MOSOMES, HEREDITY AND SEX. 
49] 
Apart; from the fact that^ according to many observers^ the 
pairing and separation of the chromosomes in the maturation 
divisions provide just the mechanism required to bring about 
Mendelian segregation^ there are two other lines of argument 
which point in the same direction. One of these is again 
inferential only, the other depends on direct observation, 
ddie former is concerned with the phenomenon of gametic 
coupling — the fact that in a number of cases two separate 
Mendelian units tend to be more or less closely associated 
in their inheritance. Gametic coupling was first discovered 
by Bateson and Punnett (6, 7) in plants. In animals it has 
been worked out rather fully by Morgan in Drosophila (41, 
etc.), and is now known to be widely distributed. The facts, 
put shortly, are that if a form possessing characters A and B 
is mated with one from which these factors are absent 
(represented a, b), the crossed individual produces gametes, 
most of which bear either both and B, or neither of 
them, the combinations A b and a B being’ relatively rare. 
The fact that there is this association in transmission between 
distinct Mendelian factors is strong* evidence that they are 
borne in some body which behaves in gametogenesis as a 
unit, and the only bodies known to behave in the way 
required are the chromosomes. This argument is strengthened 
by the behaviour of such characters in cases of sex-limited 
inheritance, which it will be convenient to discuss at a later 
stage. It is also strongly supported by the fact that in 
Drosophila there are three groups of such coupled charac- 
ters; each of the characters included in any one group shows 
coupling with the others of the same group, but characters 
belonging to different groups are inherited independently of 
one another. Morgan has put forward a hypothesis which it 
must be admitted is rather speculative, to account for these 
facts (42-44). He suggests that the factors for the coupled 
characters are all borne in one chromosome, the chromosome 
being regarded as consisting of a series of units arranged 
along its length like beads on a string, each unit bearing one 
^Mactor.” In synapsis two such chromosomes pair side by 
