124 Dr. H. Gadow. Evolution of the Colour-pattern [May 22, 



out of its normal course. But this colour increases in extent and 

 intensity for a long time after the pairing, when the females are big 

 with ripe eggs. Often the pigment is invisible externally, but it is 

 nevertheless present in great quantities in the deeper strata of the 

 skin, for instance, in C. mexicanus of both sexes. Sometimes the whole 

 peritoneum and other cavities are jet black. The abundance of these 

 waste products is stowed away where it cannot do any harm ; such a 

 favourite place are the underparts of lizards, and it is quite conceivable 

 that natural weeding out takes place wherever the heaping up of such 

 surplus pigment on the visible parts would upset an existing pattern 

 which has turned out extremely advantageous. But this does not 

 mean that selection has caused this pattern. 



A feature well started, whether by internal or by external causes, is 

 likely to proceed onwards with cumulative effect. Whatever it may be 

 that has first guided pigment to be arranged in longitudinal lines, will 

 continue to do so until a linear marshalling of pigment in the skin 

 becomes an integral, almost ineradicable habit in that kind of organism. 

 But size and proportions change with growth, blood-vessels yield to 

 new systems of supply, new modes of distribution of light and colour 

 follow with changes in the surroundings, and all these new conditions 

 may make it not only difficult to keep up the old working of the 

 organism but may even introduce new features, or improved methods. 



Some lizards cannot keep their stripes unless they live in stripe- 

 exciting surroundings ; just as little can others keep pale spots in a 

 desert. When spots are indicated in a desert creature, the spots are 

 dark on a light ground ; in the inhabitants of forests light spots on 

 dark ground are the fashion. 



An illustration of incidental advantage of features which are 

 primarily due to constitutional conditions is the following. In lizards 

 with very minute granular scales, e.g., C. deppei, the striation and colour 

 of the back appear most vividly marked when the creature is seen from 

 behind, but if looked at from the head tailwards, our eye more or less 

 in level with the long axis, then the lizard assumes an almost stripeless 

 and neutral tint. This latter phenomenon may be useful to the lizard 

 in the pursuit of its prey, just as much as the striation may be advan- 

 tageous in saving its own skin whilst running away from, and dazzling 

 its pursuer. This phenomenon results from the fact that all scales, 

 even the minutest, are more or less imbricating. 



To appreciate these effects of colour and pattern, it is as well 

 to mention, that we must contrive to look at these creatures from 

 their own perspective, in their own level, not from a bird's-eye view. 

 For instance, when lying on the ground ourselves, we appreciate the 

 concealing, or dazzling effect of the lateral spots which are such a 

 common feature in male ground-lizards ; and we also observe that their 

 more sombre-coloured females get on just as well without these " orna- 



