Increase in Number of Species 



159 



Fig. 64. Range of the smooth green snake Opheodrys vernalis vernalis show- 

 ing the small isolated population in the Black Hills of South Dakota. (After 

 P. W. Smith.) 



Fig. 65. The disjunct range of the purple skunk cabbage Symplocarpiis 

 foetidus. (After Fernald. ) 



adjacent Asia, and other pairs (matching the skunk cabbage dis- 

 tribution) have one species in Asia and the other in eastern North 

 America ( Ross, 1956 ) . In the plants a pair of North American sister 

 species of the genus Senecio are the undoubted result of range dis- 

 junction, one species now occurring in the western mountains and 



