204 MULTIPLE ALLELOMORPHS 
that a factor could change in more than one way, 
and thus give rise to multiple allelomorphs, unless 
it is supposed that the only change possible in a factor 
is a complete loss of the factor, as postulated in the 
presence and absence theory. 
There is, however, an alternative theory to that of 
multiple allelomorphism. This alternative is com- 
plete linkage. The numerical result can be equally 
well explained if, instead of occupying identical loci, 
the factors are so near together that they never 
(or very rarely) cross over. For reasons that will be 
given later we are inclined to think that the 
explanation of multiple allelomorphism is in most 
cases the more probable one, but the arguments in 
favor of this view may be deferred until the facts 
have been described. 
There is a general relation that so far holds for 
all cases in which multiple allelomorphs have been 
discovered, namely, that the factor-differences pro- 
duce similar effects. All of the following examples 
illustrate this relation. 
In rabbits (Fig. 50) the Himalayan pattern has 
been shown to behave as a recessive to self-color and 
a dominant to albino. Any two of these three types 
of pigment formation and distribution give a 3:1 
ratio in F, but no two of them, when crossed, ever 
produce the third genetic type. In other words the 
factors behave as though allelomorphic, for only two 
can be gotten into any one individual. A-similar re- 
lation has been described by Baur in the columbine, 
where three types of leaves, green, variegated (green 
