168 ANIMAL LIFE AND HUMAN PROGRESS 



If this process occurs and if the factors are arranged in a 

 linear series along the chromosomes it is clear that the nearer 



Black 

 Vestigia,! 



Long 



A 



B 



FIG. 6. Diagram illustrating the conception of " crossing over " among 

 the chromosomes, as developed by the American school. Whether crossing 

 over of the factors for " grey " and " long," and for " black " and " vesti- 

 gial," occurs, depends upon whether the point of junction and subsequent 

 breaking between the chromosomes falls in the area separating the factors, 

 or beyond it on either side. Crossing over of the factors in question occurs 

 in B, but not in A or C. 



they are together the less the chance of their becoming 

 separated. On the basis of these assumptions and of the 

 observed value of the intensity of linkage between the various 



