CHROMOSOMES AND P.VOLrTTON 10r> 



by differences in the sequence of the genes in the 

 chromosomes, and separtated })y more or less complete 

 Kterility-barriers. 



Conclusions 



In tlie liglit of modern cytoloory and genetics 

 Darwin's stat(*ment that ' varieties are incipient 

 species ' must be modificnl to n^ad ' some varieties are 

 incipient species '. We have seen that in all the 

 types of reproductory mechanism the primary origin 

 of new species lies in some accident in the chromo- 

 some set. The occurrence of such an accident is 

 entirely unconnected with natural selection, but if it 



