Increase in Number of Species 205 



mixing gene pool which is segregated locally into discrete types 

 (some of the five subspecies and possibly other as yet undetected 

 entities ) by the selective action of local environments. This concept 

 can hardly apply to occidentalis and sonorensis, a high proportion 

 of which maintain distinctive differences in male genitalia in the 

 more xeric areas of California, Oregon, and Washington where 

 their ranges overlap. Rather, these two forms suggest the situation 

 found by Blair (1955) in his Microhyla studies. 



Paucity of existing data makes it difficult to decide on the status 

 of these Culicoides subspecies. If they represent only ecotypes 

 selected from a common gene pool, the stability of certain types 

 over large and diverse areas is a real puzzle. If they represent 

 incipient species of different age levels, then younger incipient 

 species have evolved before older ones reached a stage of sexual 

 and genetic isolation— a situation not uncommon in plants but at 

 present considered unusual in animals. Then the question arises: 

 Are these entities merging or moving in the direction of evolutionary 

 independence? 



An unusually interesting situation concerns the North American 

 garter isnakes comprising the species Thamnophis elegans and its 13 

 races, which were studied by Fox (1951). This species apparently 

 consists of two "ring species," one aquatic and one terrestrial, the 

 two "rings" overlapping geographically to a large extent, yet 

 separate genetically except for a single, small area straddling the 

 Oregon-California boundary which is occupied by the race bis- 

 cutatus common to both rings. Each ring presents the problem of 

 interpreting its own complex parts, and in addition there is the 

 problem of explaining the race hiscutatus which connects the two 

 rings. Is each ring a distinct species which in this one area 

 hybridized to form a hybrid race, or is hiscutatus a little-changing 

 population of the ancestral form which gave rise to the two "rings"? 



In plants of the Potentilla glandulosa complex, the subspecies 

 hanseni is intermediate in a series of characters between the two 

 subspecies refexa and nevadensis. This suggests that the very 

 distinct but geographically restricted subspecies hanseni may have 

 arisen since the last glacial maximum as a hybrid between the other 

 two subspecies. The subspecies hanseni occurs at intermediate ele- 

 vations in the California mountains; subspecies reflexa occurs at 

 lower elevations and overlaps upward into the range of subspecies 

 hanseni for 2,000 feet, but the two do not interbreed because of dif- 

 ferences in habitat and flowering dates. The alpine subspecies neva- 

 densis occurs at higher elevations than the other two. The three sub- 



