138 THEORY OF EVOLUTION 



each side of that point, a to A and b to B, 

 should be protected, so to speak, from further 

 crossing over. This in fact we have found to 

 be the case. No other explanation so far pro- 

 posed will account for this extraordinary 

 relation. 



What advantage, may be asked, is there 

 in obtaining numerical data of this kind? 



FIG. 68. Scheme to indicate that when the members of a 

 pair of chromosomes cross (at a-b) the region on each side is 

 protected inversely to the distance from a-b. 



It is this: whenever a new character appears 

 we need only determine in which of the four 

 groups it lies and its distance from two mem- 

 bers within that group. With this information 

 we can predict with a high degree of proba- 

 bility what results it will give with any other 

 member of any group. Thus we can do on 

 paper what would require many months of la- 

 bor by making the actual experiment. In a 

 word we can predict what will happen in a situ- 

 ation where prediction is impossible without 

 this numerical information. 



