THE ORDER OF THE GENES 125 



are shorter there, ^^ distance'' in these regions is on a dif- 

 ferent scale from distance in the middle of the same 

 chromosome. Factors for crossing over have been found, 

 by Sturtevant, that change the values in certain parts of 

 the series and leave other parts unaffected. When the 

 influence of these special genes, that can be treated in the 

 same way as are all Mendelian genes, is removed, the 

 region that was affected gives its original crossover 

 values again. 



It is to be understood, then, that when we substitute 

 the idea of distance for crossing over values the term is 

 not used in an absolute sense, but in a relative sense, and 

 that it depends always on the conditions of the experiment. 

 That the genes do stand at definite levels in the chromo- 

 somes, and that in this sense they are definitely spaced, 

 seems reasonable in the light of all the evidence bearing 

 on this point; but even if they are so spaced that crossing 

 over is a function of their distance from each other in the 

 series, any influence that determines how often inter- 

 change between homologous pairs will take place would 

 give the appearance that the actual distances themselves 

 have changed. 



