OCR Text |
Show 1906.] IN MEXICAN LIZARDS. 295 towards the Isthmus through its junction with the Sierra Madre del Sur. The Atlantic and Pacific types of hot climate are juxtaposed. The Southern species meet others of the mexicanus stock which have come from the North, at least from the plateau, and they meet others of the communis-stock which have come from the West. Or may be, if we prefer it, the Southern dep>pei-group has by its northward extension crossed the Southern members of the communis-stock, which extend from Colima along the coast across the Isthmus through Yucatan to Cozumel! Indeed, we can understand why the Oaxaca-Isthmus district should be so rich in forms. It is a highway, the meeting-ground of the South and North exchange, and at the same time so diverse in bionomic conditions, any but deserts or semideserts being there represented within a small compass. The State of Oaxaca is the meeting centre of North and South, East and West conditions-a combination which occurs nowhere else in Mexico. In comparison, the rest of this large country, in spite of wonderful variety, shows far more fundamental uniformity, each of its main divisions in its way, and, as the map will show, with rarely as many as 4, more often only 3 or 2, and even only 1 kind of Cnemidophorus. These facts are eloquent testimony that the diversity of bionomic conditions is res])onsible for the various kinds of these lizards. Never mind, for the present, whether this must mean either that natural selection has weeded out those variations which do not fit in, or that the bionomic conditions have actually caused these variations. Fortunately our Cnemidophori seem to testify that both views can go hand in hand. The change of the pattern of a typical C. mexicanus from stripes to tiger-bars during its growth from youth to age shows that this change takes place side by side with natural selection, not beyond its control. Otherwise it would mean, as I have pointed out elsewhere, that all those are weeded out which in their youth do not happen to be striped, and those of the second year which do not happen to become spotted, and those of old age which do not manage to assume the cross-barred pattern ! There are no young C. mexicanus which are not striped, but no old specimens with stripes. In C. deppei the greatest number of stripes occurs in old specimens, and this fact is not due to the others having been weeded out, since many-striped young are not relatively but positively rare. If this many-striped pattern is best for this species, it is hard on the young to have to wait for it during the time that they are most in need of protection. The changes are constitutional and also caused directly by the external bionomic prevailing conditions, and some of the "protective" results are quite incidental; for instance, the fact that many a vividly striped C. deppei appears quite stripeless, monochrome dull, when seen from in front instead of sideways or from behind. This striking feature is the result of |