LINKAGE 69 
also been found in other forms than Drosophila, but 
in these cases the determining conditions and their 
effect on the various linkage values have not as yet 
been discovered. 
LINKAGE IN OTHER ANIMALS AND IN PLANTS 
Since the discovery in 1906 of linkage in sweet peas 
many cases have been found in animals and in plants. 
In sweet peas themselves two groups of linked factors 
are now known, one containing three pairs of factors 
and the other three or possibly four. In garden 
- peas there are two pairs of linked factors and two 
other cases that are doubtful; in the primrose there 
is a group of five pairs of linked factors; in the 
snap-dragon there is a group of three pairs; in 
stocks there is a group of three or probably four 
pairs. In animals, linkage, aside from sex linkage, 
has been discovered in several forms besides Dro- 
sophila, viz., in domesticated poultry by Goodale, in 
pigeons by Cole, in rats and mice by Castle, in the 
silk-worm moth by Tanaka, and in Apotettix by 
Nabours. There are, it is true, several other cases 
in which the evidence leads one to suspect that 
linkage occurs, but these are too uncertain at present 
to be included in the list. In all the above cases 
the linkage is “ partial,’ that is, a certain amount 
of crossing over takes place, at least in one sex. 
There are a number of cases of sex linkage, which, 
being only a special case of linkage, undoubtedly 
belong in the same category, but the amount of cross- 
