386 THE CHANGING GENERATIONS 



which the initial step in speciation is accomplished. This, then, is the 

 genetic and evolutionary basis for the existence of the relations expressed 

 in Jordan's rule. The different but related populations that occupy 

 adjacent ranges have descended from a single ancestral population and 

 have become different because the presence of the barrier has prevented or 

 minimized gene interchange. 



Sometimes we find apparent violations of Jordan's rule. Many instances 

 are now known of closely allied forms which occupy the same territory 

 and sometimes even the same habitat. Most of these situations have not 

 been sufficiently investigated to permit an explanation. In those that 

 have, it has usually been found that interbreeding between the two forms 

 is prevented or minimized by some ecological, physiological, or psy- 

 chological barrier that takes the place of the geographic barrier. Even 

 so, it may be that such related forms living in the same area arose through 

 geographic isolation and acquired their reproductive isolating mechanisms 

 before coming together again. 



Here we conclude our account of the evolutionary principle and its 

 consequences. In the chapters which follow we shall trace the evolutionary 

 history of organisms from earliest times to the present, with special 

 emphasis on those phylogenetic lines which have led to man. 



