DISTRIBUTION OF THE CHROMOSOMES 141 



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 

 Pygsera 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 



