74 Comparison of Maps. 



In figure 17 we have represented the same maps as shown in figure 

 16, but drawn to different scales, so that they are roughly propor- 

 tional to the lengths of the respective chromosomes. This tends to 

 eliminate the differences due to any general increase or decrease in 

 crossing-over, but of course it does not eliminate differences that 

 affect particular regions. 



A comparison of these maps shows a degree of conformity between 

 the loci of possible "parallels" that harmonizes in most cases with 

 the view that they may be homologous. There is, of course, a 

 considerable element of chance entering in here, and some of the 

 resemblances may be accidental, but they can hardly all be accounted 

 for on this basis. 



The autosomal maps of D. virilis are not yet sufficiently reliable 

 for comparison. In considering rates of crossing-over, however, 

 we may compare the X chromosome map of virilis with the autosomal 

 maps of D. melanogaster (the only ones available for the purpose). 

 When this is done we find the same sort of difference as noted in 

 the case of the X chromosomes of the two species, i. e., crossing-over 

 is more frequent in virilis. The X-chromosome map of virilis slightly 

 exceeds both of the long autosomal maps in melanogaster, although 

 the autosomes themselves are both of the long V-shaped type, approxi- 

 mately twice as long as the rod-like X chromosome of virilis. 



