214 



THE AMERICAN NATURALIST 



[Vol. LIV 



dence shows that as much crossing over as this does not 

 usually occur. In our diagrams (Heredity and Sex, 

 1913), therefore, we represent only one or two real inter- 

 changes between the members of a pair of chromosomes 

 because the genetic evidence shows, as stated, that, in the 

 great majority of cases, this is what takes place. 



The rings in two planes, as represented in Janssens's 

 diagram, call for, further analysis. We may call these 

 rings Bb, Cc, Dd, Ee (Fig. 1 A). It will be observed that 



