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Show 294 DR. H. GADOW ON EVOLUTION [Mar. 20,. separating from C. immutabilis and guttatus, which are very different from C. sexlineatus. C, in another direction, turns in Michoacan and the Balsas basin into C. mexicanus, which reaches its terminal development in Oaxaca. Lastly, C. bocourti represents a third offshoot from C. communis (cf. p. 287 and pp. 356-358). W e have therefore to search for another explanation. W e postulate the existence of an indifferent stock, somewhat like a, combination of C. deppei and C. sexlineatus, with a range from South to North over those parts of the country which at that unknown period did not yet exhibit the present variety of physical, bionomic features. Certainly the Western Sierra was there -in bulk, but not as it is now. Then came the physical changes : subsidence of much of the Pacific land ; the development of desert features in the Northwest and North; the transformation of the central lake into a silted-up plain, the central tableland; the spreading of forests over the Sierra after the volcanic eruptions had subsided,-in short, the assumption of the more recent features of Mexico. Hand in hand with these changes went the making of the species, in loco; and as they spread further upon new ground, they changed further, giving rise to still newer varieties, races, subspecies, and species, a seemingly endless kaleidoscopic process. This is not a new process ; it was always going on, but we see only the present results, and of the many extinct forms we know naught. Broadly speaking, there are now three or four main groups. One centres round C. tessellatus, essentially in the North-west and t^orth; a second comprises the gularis-group of the Centre and West; thirdly, the essentially Southern, tropical deppei-immutabilis group; lastly, C. sexlineatus, the least specialised, in the United States. Each of the four geographical regions or main habitats of these groups has its own characteristic features ; they are types of bionomic conditions. The greatest number of well-distinguished forms occurs in the Lower Californian district. At least 6 (or 7 with C. sexlineatus in California); 4 of them are insular The existence of 3 species on the little Cedros Island is only partly an illustration of the effect of isolation-C. labialis on Cedros Island, C. rubiclus only on S. Margarita Island; maximus, tessellatus, martyr is, and hyperythrus occur on islands but also on the neighbouring mainlands, which consequently prove to be veritable refuge-islands, remnants of a formerly larger land-complex. Characteristic of, even peculiar to, this land are C. labialis and C. hyperythrus, whilst the other species are of tessellatus descent. Another centre of great variety is Oaxaca, with likewise 6-7 forms which are referable to the deppei and the gularis-communis-mexicanus group. This great diversity is in concord with the very varied physical features of that State. It is there that the Tierra caliente embraces with an eastern and a, western arm the most southern portion of the plateau, a wedge which is continued |