Increase in Number of Species 



197 



Fig. 86. Cross-breeding chart between various populations of Culcx pipiens. 

 El-Al, western Europe; Os, Osterberg; Tu, Tubingen; USA, Illinois; fat, Cali- 

 fornia; Tu, Tunis; Sc,La, Italy; Og, Fi\ Oggelshausen, Prague; Ha, Im, 

 Hamburg, Immenrode. The heavy line indicates cross fertile in both direc- 

 tions; the thin line indicates cross fertile in direction shown by arrow; the 

 broken line indicates cross sterile in direction shown by arrow; the dotted line 

 indicates cross sterile in both directions. Populations within same circle are 

 completely compatible genetically. (After Lavan.) 



species range. The present system of species boundaries can be 

 explains more logically on the basis of range movements associated 

 with the last ice age. During that time the range of the parent 

 species could have become divided into several or many isolated 

 units, and in each segregate mutations leading to these intersterility 



