158 U.S. NATIONAL MTJSEUM BULLETIN 259 



ecological isolating mechanisms between these two species, as already 

 suggested. This phenomenon is apparently not uncommon in areas 

 where original ecological isolation between two species has been broken 

 down by destruction of their habitats by man (Mayr, 1964; Mayr et 

 al., 1953). 



The rarity of S. hayi, and the fact that this species is sympatric 

 with S. t. potomacus, which is far more common, may indicate the 

 inability of the former to compete successfully with the latter. The 

 possibility that S. hayi is more cryptic and has, therefore, been able 

 to elude capture, must be considered, although this possibility does 

 not seem to offer an especially acceptable alternative explanation. 

 It should be remembered that populations of S. pizzinii are also rare 

 in the Washington area, and this species may, similarly, be unable to 

 compete on an equal basis with S. t. potomacus. 



Migration of populations of preciu'sor hayi into the Washington 

 area probably predated migration into this area by populations of 

 precursor tenuis. More recent colonization of similar ground-water 

 niches by the vagile and apparently ecologically successful S. tenuis 

 or its immediate past ancestor would account for the sympatry of 

 these two species as weU as for the present rarity of S. hayi. The 

 distinct possibility that S. hayi is the relict of a species previously 

 more widespread and that it is now on the verge of extinction should 

 be given careful consideration. 



Four species — S. alahamensis, S. balconis, S. hijurcatus, and S. 

 montanus — share so many morphological similarities that they must 

 be regarded as having had a close common ancestry. Two other 

 species, S. barri and *S'. reddelli, while differing from the first four 

 species by one principal character each, were probably also derived 

 from the same parental stock. It is possible to account for the 

 speciation and present distribution of foiu", if not all six, of these 

 species by supposing a widely distributed, vagile ancestral form, which 

 once ranged throughout most of the range now occupied by S. ala- 

 hamensis s. lat. and which extended southwestward through the 

 Arbuckle Mountain region of southern Oklahoma into central Texas. 

 Owing to the rather imspecialized morphology and wide range of 

 contemporary alahamensis, it is doubtful whether this species pres- 

 ently differs much from the suggested hypothetical ancestral form. 

 For this reason, as well as for purposes of discussion, this postulated 

 ancestral form wUl be referred to as alahamensis stock. 



Geologically, the most ideal time for wide dispersal of populations 

 of alahamensis stock would have been during one of the suggested 

 periods of Ozark peneplanation. Fenneman (1938) and Bretz (1965) 

 presented evidence for several uplifts and peneplanations of this 

 general region during the Cenozoic, the last of which may have oc- 



