WHITE— STUDIES OF INHERITANXE IN PISUM. 583 



Further, on the chromosome-Hnkage-factor-crossing-over hy- 

 pothesis, the amount of variation in a group of similar organisms 

 (a species), ehminating that caused by environmental changes, 

 would be in proportion (i) to the number of factor differences 

 between the various forms or varieties of the group; (2) to the 

 number of pairs of chromosomes; (3) possibly to the relative sizes 

 of the chromosomes (small chromosomes making crossing-over much 

 more difficult) and (4) to the amount of cross fertilization which 

 took place (either natural or artificial). 



As Morgan points out, the interpretations of the genetic data on 

 Drosophila crosses advanced by him and his students hold whether 

 the chromosomes are or are not the bearers of the factors. 



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