1906.] ly MEXICANf LIZARDS. 295 



towai-ds the Isthmus through its junction Avith the Sierra Madre 

 del Sur. The Atlantic and Pacific types of hot climate are juxta- 

 posed. The Southern species meet others of the mexicanus stock 

 which have come from the Xorth, at least from the plateau, and 

 they meet others of the commanis-iitoc\i which have come from 

 the West. Or may be, if we prefer it, the Southern deppei-yrouiy 

 has by its northward extension crossed the Southern membeivs of 

 the communis -stock, which extend from Colima along the coast 

 across the Isthmus through Yucatan to Cozumel ! Indeed, we can 

 understand why tlie Oaxaca-Isthmus district should be so rich in 

 forms. It is a highway, the meeting-ground of the South and 

 Xorth 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, ajid, as the map will show, 

 with rarely as many as 4, more often only 3 or 2, and even only 

 1 kind of Cnemido2y]iorus. 



These facts are eloquent testimony that the diversity of hionomic 

 conditions is resjjonsihle 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. Foi'tunately our Cnemidojyho^'i seem to testify that 

 both views can go hand in hand. 



The change of the pattern of a typical 0. ^nexicanus 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 conti-ol. 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. dejypei the greatest number of stripes occurs in old speci- 

 mens, and this fact is not due to the others having been weeded out, 

 since many-striped young are not I'elatively but positively rare. 

 If this many-striped jjattern 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 causeddirectly 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. depjyei appears 

 quite stripeless, monochrome dull, when seen from in front instead 

 of sideways or from behind. This stiiking feature is the result of 



