8 BULLETIN 1489, U. S. DEPARTMENT OF AGRICULTURE 
stituting the mechanism by which inheritance is accomplished. This 
similarity is brought out by a comparison of Figures 1 and 3. There 
are, however, many more heritable factors than there are chromo- 
somes. Consequently, the chromosomes must be considered as carry- 
ing these factors, rather than as the individual hereditary units 
themselves. 
LINKAGE 
It has been shown for a number of species that the chromosomes 
have characteristic shapes and sizes which persist through the cell 
divisions and which can be identified from generation to generation. 
That is, the chromosomes possess individuality, and it therefore 
seemed probable that any given factor or factors carried by a certain 
chromosome at one time would tend to remain with that chromosome 
and that characters controlled by factors located in a single chromo- 
some would tend to behave as a group. That this is the case has 
been determined experimentally. 
Factors that are located in the same chromosome, as judged by their 
behavior in transmission, are said to be linked, and the phenomenon 
is called linkage. There is a tendency for linked factors to come 
out of a cross in the same combination as that in which they entered 
it in the two parents. It should not be understood, however, that 
linkage need be absolute. The strength of the linkage tendency 
differs with the individual factors. 
The linkage relations of a number of factors in corn have been 
determined, and the phenomenon obtains here as in other species 
(#, 54). Obviously, however, there can be as many linkage groups 
as there are pairs of chromosomes, and the determination of these 
groups is increasingly difficult as the number of chromosomes in- 
creases. It therefore is better to use an example from a form with 
fewer chromosomes than corn. 
Because it is peculiarly adapted for the purpose, extensive genetic 
experiments have been carried on with the common fruit fly. This 
has but four chromosomes, and it has been possible for Morgan and 
his associates to demonstrate that the more than 300 factors for 
which they have determined the mode of inheritance fall within 
four distinct linkage groups. Moreover, the number of factors in 
each group corresponds to what might be expected^ from the relative 
sizes of the four chromosomes. Even more convincing is the evidence 
from an association of certain factors in inheritance with specific 
chromosomes which are concerned intimately with the determina- 
tion of sex and with certain aberrant chromosome behavior. This 
need not be discussed here. It is enough to say that the similarity 
of the behavior of simple characters in inheritance to that of the 
chromosomes points to the latter as the mechanism of heredity, and 
that all evidence from more complex cases and from more detailed 
study supports and strengthens this theory. 
INHERITANCE IN SELF-FERTILIZED LINES 
Because of the importance of selection within self-fertilized lines 
as a method of corn improvement, it is advisable to consider also 
the mode of inheritance under self-fertilization. The principles are 
