1903.] and Orthogenetic Variation in Lizards, etc. 121 



and then cross-barred C. guktris and C. tesselatus of Northern Mexico 

 and of the Sonoran sub-region. But with no precise data for the 

 natural conditions of these creatures, any further speculation would be 

 unprofitable. 



There is a continuous line of development leading from a completely 

 striped form like C. deppei to another, C. striatus, in which the white 

 stripes show an unmistakable tendency of breaking up into spots, until 

 there are only spots left in C. guttatus, and this phenomenon means in 

 this case the gradual preponderance of dusky pigment. On the other 

 hand C. deppei, itself undoubtedly a descendant of a 6-striped form, 

 has the tendency to increase the number of its stripes. 



I connect these changes with the different features in the distribu- 

 tion of light which prevails in the various terrains on which these 

 lizards live. Notice the peculiar distribution of sunlight and shade 

 prevailing on the ground where strong light falls through dense 

 vegetation ; there are not many shadows, but plenty of shade, and 

 this is interrupted by the familiar little sun images. Contrast this 

 with the sharp alternation of bright light and shadows under a tuft 

 of grass on sandy soil in glaring sunshine, where one blade reflects 

 white light while the shady side of its neighbour appears green or 

 dark by contrast. Again, observe the criss-cross shadows under a 

 shrub scantily dressed with leaves, as typical of dry tropical 

 countries. 



My contention is that light influences the distribution of pigment 

 in the skin. The primitive pattern is one of longitudinal stripes, and 

 at least some of these are intimately connected with the distribution 

 of primitive blood-vessels, e.g., the so-called epigastric veins. How 

 a few stripes can give rise to more is easily observed : the pig- 

 ment in the intervening dark bands is not evenly distributed, but 

 collects near the margins, the centre becomes lighter and ultimately 

 the uniform band is transformed into two dark and one new light 

 stripe. This is a common phenomenon in lizards, r/., C. deppei and 

 Werner.* The production of a white band by confluence of white 

 spots which have appeared in a dark field, is fundamentally the 

 same process, cf. Arneiva and Werner fig. 58. Longitudinal stripes 

 then, being referable to the preformed course of blood-vessels ; are 

 natural, and we can imagine how they are emphasised or stimulated 

 by the skin being continuously submitted to linear lights and shades. 

 The foundation for their direction was laid by the blood-vessels ; 

 oblique markings are out of the question in bilateral creatures, but 

 transverse markings are quite possible, even to be expected, but they 

 cannot be produced directly out of longitudinal lines. These have first 

 to be broken up and the fragments have to be recombined. This is 



* " V'ntersuchungen iiber die Zeichnung der Wirbelthiere," ' Zool. Jahrb.. 

 Syst. 1 VI. (1892), p. 155229, Plates 610; in particular, Plate 9, figs. 44 and 45. 



