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Show 306 DR. H. GADOW ON EVOLUTION [Mar. 20, Femoral pores less than 18. Collar composed of large scales (see above). No pale field-spots. 6 white stripes and a mid-field with 1 or 2 pale ^tripes. Male with white under parts. It will be found from examination of the tabular statistics of G. deppei, cf. p. 315, that such C. deppei actually occur in the lowlands of South Guerrero, notably between the coast and San Luis Allende. Specimens from other countries are ruled out of comparison either by their numerous stripes or femoral pores, &e. There is in particular one of the specimens from Cocoyul (text-fig. 73 C and text-fig. 65 E) wrhich should be a fair test case. It happens to be a young male of 63 mm., and the only obvious differences from C. sexlineatus of Sauz (text-fig. 73 A, B) are the sharply marked black of the collar, and that the 4th supraocular is represented by a tiny remnant only. Since it is in this district of Guerrero that the 4th supraocular crops up not unfrequently, females, without the criterion of the black under parts, may easily appear undistinguishable from typical C. sexlineatus. I consider this an example of extraordinary convergence of two perfectly " good species" which nowhere are known to commingle. Specimens of one clan of the northern species, in the very north of Mexico, and specimens of one clan of the tropical, southern species in South Guerrero, have hit upon the same combination of numerous characters so as to have become practically not distinguishable ! There is scarcely a greater difference in physical features imaginable than between the wind-swept, droughty arid plateau of Sauz and the hilly woody lowlands of tropical Guerrero. It is inconceivable that members of the same species of Cnemidophorus (lizards which so obviously react upon the physical features of their surroundings) should vary in exactly the same direction in such absolutely different places. On the contrary, we must conclude that the two clans are of different species; in other words, that these Sauz and Guerrero specimens are an illustration of convergence. At the same time, without attempting to quibble, wre may consider it fairly proved that C. sexlineatus and C. deppei are very closely related to each other, so intimately that they might be considered as the two divergent races of one species, upon the ground that, taken in a lump, they have now been shown to " run into each other." Only, there are these grave difficulties. First, they do not live together but are hopelessly separated. Secondly, C. sexlineatus forms the starting-point for species like C. gularis with strongly enlarged forearm-scales and a strong collar, while the tendencies of C. deppei are the increasing number of stripes, black pigmentation of the males' underparts, and loss of the 4th supraocular. Lastly, from some form like G. deppei have been developed C. immutabilis and C. guttatus,lizards which have radiated in a direction opposite to that of the "descendants" of C. sexlineatus. |