THEORY OF EVOLUTION 143 



We have found practically no factors less 

 than .04 of a unit apart. If our map includes 

 the entire length of the chromosomes and if we 

 assume factors are uniformly distributed along 

 the chromosome at distances equal to the short- 

 est distance yet observed, viz. .04, then we 

 can calculate roughly how many hereditary 

 factors there are in Drosophila. The calcula- 

 tion gives about 7500 factors. The reader 

 should be cautioned against accepting the 

 above assumptions as strictly true, for crossing- 

 over values are known to differ according to 

 different environmental conditions (as shown 

 by Bridges for age), and to differ even in dif- 

 ferent parts of the chromosome as a result of 

 the presence of specific genetic factors (as 

 shown by Sturtevant). Since all the chromo- 

 somes except the X chromosomes are double 

 we must double our estimate to give the total 

 number of factors, but the half number is the 

 number of the different kinds of factors of 

 Drosophila. 



