188 DISTRIBUTION OF THE CHROMOSOMES 
as those of the chromosomes. Finally, the distribu- 
tion of the factors within any one group is what the 
chromosome hypothesis calls for. For the fre- 
quencies of separation (or combination) between the 
different factors of a group are in a linear relation to 
each other, and the relation is even specifically of 
such a type (involving interference) as would be 
expected to occur if the separations between the 
factors resulted from the crossing over between two 
twisted chromosomes which the cytological evidence 
indicates may occur. 
Even in cases where the chromosomes are not 
distributed in the usual way it is found that the 
factors have the same unusual method of distribu- 
tion. For example, in moths there are some cases of 
extraordinary interest because the chromosomes can 
be traced to and through the ripening period of the 
eggs of the hybrid. Certain species of the moth 
Pygera that have different numbers of chromosomes 
were crossed by Federley. The full number (calcu- 
lated) and the reduced number of chromosomes in 
the different species are as follows: 
Diploid Haploid 
P. anachoreta 60 30 
P. curtula 58 29 
P. pigra 46 23 
In the hybrids, the full number is the sum of the two 
haploid sets that went in from the parents. This 
shows that the chromosomes preserve their individ- 
uality through many successive cell divisions in a 
