TEIID LIZAT^DS OF THE GENUS CNEMIDOPIIOKUS 73 



Enemies. — The Mexican whip snake, Mastlcophis menfovarius, 

 was mentioned by Gadow (1908, p. 166) as preying upon these 

 lizards. 



Aifinitics. — Because of its geographical and structural identity 

 Avith depp-ii and its marked ditFerences from gula7^s, its only other 

 possible ancestor, giittatus is here considered as a direct derivative of 

 deppU^ or, perhajis more properly, of ancestral deppii-Itke stock. 

 The occurrence of both guttatus and deppii in the same region is 

 contrary to the usually applicable rule that directly related forms 

 are not found in the same locality. This occurrence would seem, at 

 first sight, to indicate speciation by mutation alone as suggested by 

 Blanchard (1921, p. 253) for Lampj'opeltis callforniae calif omiae 

 which is found in southern California in coextensive distribution 

 with LampropcJtis getulus hoyVtl^ from which it is apparently de- 

 rived. For these Californian snakes, mutation alone, Avithout isola- 

 tion, seems to be the only possible means of speciation, but for the 

 forms found in the region of Central America and southern Mexico, 

 such as deppii and guttatus in this case, it is not necessary to resort 



D — >G| G 



D »d1 D 



io"n Iso\aUor> Un\oT\ M\graUoTi an^ IniermixigUng 



Figure 17.- — -A theoretical diagram of the maxnee in which two closely re- 

 lated SPECIES, at present IN CO-EXTENSIVE DISTRIBUTION, MAY HAVE EVOLVED 

 FROM A COMMON STOCK THROUGH ISOLATION. B AND D' = DEPrii; G = GUTTATUS 



wholl}^ to this explanation. The theory of speciation by isolation 

 (during which mutation has an oportunity to become of constant 

 nature and hence of diagnostic value) may be used here. (See 

 fig. 17.) 



It has generally been conceded by geologists (see Schuchert, 1929, 

 p. 141) that the Tehuantepec region was lowered to below sea level 

 in the post-Miocene or early Pliocene and that the resulting water 

 barrier kept Central American forms isolated from those in southern 

 Mexico for a significant length of time. 



For reasons to be presented in the general discussion at the end 

 of this work it is presumed that the ancestral deppii stock became 

 modified from the ancient lemniscatus stock during the late Miocene. 

 Once differentiated, this generalized type spread northward and 

 probably entered southern Mexico before the formation of the 

 Tehuantepec portal. After the subsequent isolation and upon the 

 re-formation of the Tehuantepec land bridge, later in the Pliocene, 

 there is the possibility of a secondary union of the noi'tliern and 

 southern faunal elements. 

 2307—31 G 



