THEORY OF EVOLUTION 137 



If we assume that the nearer together the 

 factors lie in the chromosome the less likely is 

 a twist to occur between them, and conversely 

 the farther apart they lie the more likely is 

 a twist to occur between them, we can under- 

 stand how the linkage is different for different 

 pairs of factors. 



On this basis we have made out chromosomal 

 maps for each chromosome (fig. 67) . The dia- 

 gram indicates those loci that have been most 

 accurately placed. 



The Evidence from Interference 



There is a considerable body of information 

 that we have obtained that corroborates the lo- 

 cation of the factors in the chromosome. This 

 evidence is too technical to take up in any de- 

 tail, but there is one result that is so important 

 that I must attempt to explain it. If, as I 

 assume, crossing over is brought about by twist- 

 ing of the chromosomes, and if owing to the 

 material of the chromosomes there is a most 

 frequent distance of internode, then, when 

 crossing over between nodes takes place at 

 same level at a-b in figure 68, the region on 



