THEORY OF EVOLUTION 129 



Yellow Gray Yellow Gray 



White Red Red White 



99.% 



Not only have the two grandparental combi- 

 nations reappeared, but in addition two new 

 combinations, viz., grey white and yellow red. 

 The two original combinations far exceed in 

 numbers the new or exchange combinations. If 

 we follow the history of the X chromosomes we 

 discover that the larger classes of grandchildren 

 appear in accord with the way in which the X 

 chromosomes are transmitted from one genera- 

 tion to the next. 



The smaller classes of grandchildren, the ex- 

 change combinations or cross-overs, as we call 

 them, can be explained by the assumption 

 that at some stage in their history an inter- 

 change of parts has taken place between 

 the chromosomes. This is indicated in the 

 diagrams. 



The most important fact brought out by the 

 experiment is that the factors that went in to- 

 gether tend to stick together. It makes no 

 difference in what combination the members of 



