120 BULLETIN 15 4, UNITED STATES NATIONAL MUSEUM 



'Enemies. — The prairie racer, MmticopJus fagellum favigulcuis^ 

 has been listed by Strecker (1927, p. 10) as preying upon the spotted 

 race runner. 



Ajfinities. — For reasons to be presented in the general discussion at 

 the end of this work (pp. 251-260), gularls., the prototype of the 

 sexJmeatus group, is thought to have been directly derived from a 

 common ajicestral stock Avith the deppl and tessellatus groups, prob- 

 ably in southern Mexico. Its closest modern relatives are not these, 

 but sexlmeatiis and perpleiicus. 



The intergraclation between gularis and sexUneatus is unusually 

 extensive and takes place in southeastern and southern Oklahoma 

 and in much of Texas — from the panliandle southeast to the mouth 

 of the Eio Grande. Part of these intergrades show the blue ventral 

 suffusion and the enlarged postantebrachials of gularis.^ while the 

 rest show the ventral whiteness and the granular postantebracliials of 

 sexUneatus. These major variations occur independently of each 

 other and in all manner of combinations. Intergradation also occurs 

 in spotting and in the maximum size of individuals. 



Brown (1908, p. 548) wrote that ''while sexUneatus and gularis 

 have doubtless had a common origin, the differentiation reached in 

 the adults is great, and the occasional retention of the earlier style 

 of color marking by the young of gularis can not be regarded as true 

 intergradation." Under this supposition, it would be inferred that 

 gularis has been derived from sexUneatus, but for reasons to be pre- 

 sented later, this does not seem to be the case. Many specimens from 

 the Mexican mainland differ from sexUneatus only in the presence of 

 enlarged postantebrachials, others only in the presence of a bluish 

 ventral suffusion, and still others only in the presence of spots in 

 the lateral fields. Fortunately, perhaps, all Mexican specimens ex- 

 amined by the Avriter have been found to differ from sexUneatus in 

 one of these ways, but these variations or combiantions suggest 

 sexUneatus long before its actual evolution in the United States. 

 The significant thing about the intergradation in Texas is not that 

 the young of the two forms are often alike but that there is an exten- 

 sive overlapping or confusion in the structural and colorational 

 features of the adults as well. 



The intergradation between gularis and perplexus is apparently 

 not as extensive as that between gularis and sexUneatus, although the 

 line of contact between the two forms is fully as long, extending 

 from the panhandle of Texas southward to Coahuila and west to the 

 Gulf of California. This difference may be attributed to the rela- 

 tively poor delineation of contrasting habitat conditions at the line 

 of contact between gularis ancl sexUneatus. 



