COLORATION AND ITS INTERPRETATION 77 



transverse direction, accompanied by the deposition of dark 

 pigment along the forming borders, leads to a pattern of trans- 

 verse bars, corresponding to the third stage in the colour-evo- 

 lution of the wall-lizard ; while by the disappearance of both 

 spots and bars the final uniform coloration is reached. 



In these American lizards, as in the wall-lizard, there ap- 

 pears to be no doubt that the longitudinally striped condition 

 was the original one ; the spotted type being a secondary con- 

 dition, from which have been evolved the cross-barred and 

 finally the uniformly coloured stage. And as these changes 

 are more or less paralleled both among birds and mammals, it 

 may be definitely accepted that stripes and spots are more 

 ancient than a uniform colour. 



As regards the reason for these changes, it has been ob- 

 served that if we start with the original and typical form, 

 marked with six strongly pronounced white stripes, it will be 

 found that in individuals frequenting sandy districts dotted with 

 sparse tufts of grass there is a tendency to an increase in the 

 number of stripes, of which there may be from eight to eleven. 

 On the other hand, in neighbouring spots, where the vegetation 

 is more abundant, the number of stripes is usually from seven 

 to nine, and these show a tendency to break up into spots on 

 the hind part of the back. As we pass into open tropical forest 

 with much undergrowth the lateral stripes fade, while the 

 others dissolve into spots which have a tendency to disappear 

 on the loins in old individuals. Again, in races inhabiting open 

 table-land with scattered spiny shrubs and hedges, the young 

 form with six light stripes and pale spots on the dark bands 

 tends to pass into a cross-barred type. Finally, in similar 

 localities to the last, but with more mixed vegetation, all the 

 lines become broken up into spots, in addition to those which 

 existed in the intermediate dark intervals. 



These changes appear to be correlated with the varying 

 distribution of light in the different stations these lizards re- 

 spectively frequent. If this be so, the colour-changes are 

 protective in their nature ; and it has been urged as a reason 

 for the frequent elimination of the original white striping that 

 the effect of such an arrangement is constantly interfered with 

 by the movements of the animal. Further, it has been pointed 

 out that it is far more natural for the lights and shadows to fall 



