1906.] IX MEXICAX LIZARDS. 307 



The principle here involved is to a certain extent expressed by 

 the homely saying, " what is one man's meat is another man's 

 poison." It may be expressed by the following equations : — If 

 X and y are two lizards in an indifferent state, or before they 

 have been subjected to very different modifying oecological con- 

 ditions, A standing for Plain, B for Forest, and R the result, 

 then xA = R and yB=R, i. e. xA can only be =yB if x and y are 

 different, i. e. reacting differently ; it being also inconceivable 

 that the same kind of creature, if modified at all by the absolutely 

 different factors A and B, should be modified into the same 

 combination of characters. 

 xA = yB. 



, i. e. X = 2nd species as it would be if adapted for 



Forest life, but modified by the Plain. 

 , i.e. y — 1st species as it would be if adapted for 



Plains, if it were not modified by Forest life, 

 for argument sake, assume that Plains favour the 

 development of scutes on the forearm, 4 supraoculars and few 

 pores; and that Forest life increases the number of pores, while 

 it disallows or destroys scutes. Then our equation would mean : 

 a; = a Forest species which has been changed into one foi- Plain 

 life ; i. e., it has develojjed arm-scutes, retains all the 

 supraoculars but requires few j^ores. 

 2/ = a Plain species which has been adapted to, or changed by, 

 Forest life; i. e., scutes are reduced and pores are increased. 

 In other words, x and y, the original stocks of C. sexlineahcs 

 and C. depjiei, must have been different. 



On the other hand, to assume x = y would imply that A:=B; 

 physical conditions which we started with as being opposite to 

 each other. 



OsrEMIDOPHOfiUS HYPERYTHRUS Cope. 



Cope, Proc. Am. Phil. Soc. 1869, p. 159, established the genus 

 Verticaria for those Cnemidophorus-like species which are dis- 

 tinguished by the fusion of the two frontoparietal plates into one. 

 Such species are C. heterolejyis Tschudi, from the neighbourhood of 

 Lima, Peru, and C. hyperythrus Cope, from Lower California, in 

 which I include, following Boulenger's advice, C. sericea van Den- 

 burgh and C. heldingi. Hedracantha Bocourt is, as Boulenger 

 has shown, not a Cnemidojyhorus but an Ameiva, and does not 

 occur in Mexico as stated erroneously by Bocourt and Coj)e, but 

 near the coast of Peru and Ecuador. The fact that the fusion of 

 the originally double frontoparietals occurs in two different 

 geneia, and the unique scaling of C. heterolepis, appear sufficient 

 to disallow the fusion as a geneiic character. I am inclined to 

 look upon these few " Verticarias " as remnants of a more Western, 

 Pacific fauna, and in my paper Proc. R. S. 1905, I have given 

 reasons which indicate a former westward extension of Mexico 

 and Central America. 



Proc. Zool. Soc— 1906, Yol. I. No. XXI. 21 



