282 ANIMAL DEFENCES 



Examples of these are the Thorn-tailed Lizards (Uromastix] of 

 North Africa and South-west Asia, which are burrowing forms 

 living entirely on plant-food, so that the coloration may be 

 regarded as entirely protective, while in species which prey upon 

 insects and other small animals it is aggressive as well. This 

 is the case, for instance, with the Desert Monitor ( Varamis 

 griseus), which has pretty much the same range as the last- 

 named species. It is a large animal, attaining a length of over 

 4 feet, and presents dark bars and streaks upon a neutral ground- 

 colour. Quite similar as regards general hue and character of 

 marking is the Common Skink (Scincus officinalis] of the Sahara,, 

 which, however, is only about 3 inches in length. 



Our only poisonous British Snake, the Adder (Pelias berus} 

 (see vol. i, p. 233), commonly found on sandy heaths, exhibits 

 general resemblance to surroundings, which is both protective 

 and aggressive, as in the case of the Sand- Lizard. The ground 

 colour is usually brownish or greenish gray, and the back is 

 ornamented with a dark zigzag stripe. There are many groups 

 of true desert Snakes which are as inconspicuous as the Mammals 

 and Birds which inhabit the same regions, and here, too, as in 

 the Adder, the colouring serves a double purpose. A typical 

 example is the Horned Viper (Cerastes cornutus), receiving its 

 name from the presence of a pointed projection above either 

 eye in the male, and less often in the female as well. It inhabits 

 the deserts of North Africa and Arabia, and is of a sandy colour 

 with numerous dull blotches. 



Desert Insects also exhibit the dull neutral tints and blotchings 

 or mottlings which prove so efficient in vertebrate forms. Ex- 

 amples are found in certain Desert Grasshoppers (Xiphocera asina y 

 Trachypetra bufo, and Methone Anderssoni] native to South Africa. 



REVERSED SHADING. General protective resemblance is not 

 merely a matter of appropriate colouring, but also of appro- 

 priate disposition of light and shade, and this is exemplified by 

 many of the cases already described, as well as by others to which 

 allusion will now be made. If such a curved white or brownish 

 solid as a common hen's egg be placed on a table and strongly 

 illuminated from above, it will be obvious that its upper part 

 will present high lights, while its under side will be in shadow, 

 the two regions gradually merging into one another. When a 

 drawing of an egg is made on a piece of paper, the intention is 



