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Show 1906.] IN MEXICAN LIZARDS. 289 of 1200 miles asunder. C. deppei and C. sexlineatus, with regard to each other, are two good species in the fullest sense, although all their available characters may overlap, or intergrade, not only singly, but conjointly. They are two old species, sprung from one common stock, well and firmly established, representing each other in widely separate and apparently very different countries, one in the Tropics, the other in North America proper. Florida and Texas have much of the type of the Tierra caliente, but it would be hopeless to look for the tertium comparationis between the more Northern States and the Tropics of Mexico and Central America, unless we assume that the North-western Plateau, with its ranges of mountains, from the Western States right through Mexico, has caused the evolution of the many other kinds of Cnemidophori, which now separate and connect C. deppei and C. sexlineatus*. Our problem is not to explain why these two species should occasionally be so much alike each other in their widely different habitats, but to investigate whether, how, and why the intervening country, the bulk of Mexico, has turned its lizards into what they are, namely the great gularis-group. The family of the Tejidae is old. Of several dozen so-called genera in South America, only two are found also in Mexico; namely, one species of Ameiva in the eastern and western hot-lands, and the genus Cnemidophorus incl. Verticaria. This genus is old, but not old enough to occur on the West Indian Islands, a fact which limits it to the end of the Miocene epoch. Yucatan was under water until the beginning of the Pliocene; it has received its few Cnemidophori since that epoch, and the same applies to the Atlantic lowlands along the Gulf to Florida. Only C. guttatus and C. deppei have got into these parts of Mexico. For Texas only C. sexlineatus and the little C. gularis were available immigrants. C. mexicanus, a very distinct species, exists on the Tres Marias Islands. Other species inhabit the small islands of Lower California, both in the Gulf and to the west of the peninsula; proofs of the existence of the genus in Mexico in early Pliocene times. It is doubtful when the great central plateau between the Eastern and Western Sierras Madres became dry land; until late Tertiary times it was an inland lake. Longest available for terrestial creatures were Southern Mexico and the Pacific portion- a great stretch of land from Central America to California, including parts of the present Pacific Ocean. It is in this belt that we have to look for the home of the Mexican and North- American Cnemidophori. Their present distribution agrees well with this hypothesis. There is an abundance of species in the South and in the North-west, whilst towards the North and East, across the plateau, occur far fewer forms. The great TESSELLATUS-group is an illustration of a group centred * Lack of material has prevented me from corroborating Cope's statement that C. sexlineatus and C. gularis absolutely merge into each other, cf. p. 305. |