THE WONDER OF LIFE 539 



{d) Another direct utility may be recognized in the pigmen- 

 tation of the skin in various animals. Thus the dark 

 skin of some animals from very warm countries and 

 the whiteness of some animals from very cold countries 

 may have a direct physiological value to its possessor. 

 It appears that the dark insoluble melanin pigments, as 

 in the crow and negro, are protective against the ultra- 

 violet rays of sunhght. A remarkable fact was observed 

 by Engelmann in regard to the pecuhar restless Algse 

 known as Oscillatoria. He found that in red hght they 

 had a green colour, and in green hght a red colour — in 

 both cases the physiologically best colour. 



Protective Value of Coloration. — Some of the finest 

 instances of adaptation are seen in the way animals resemble 

 their habitual environment. Shape and pose sometimes 

 conspire with coloration to give the animal a mantle of 

 invisibihty. Eeferring for details to books on animal color- 

 ation by Professor E. B. Poulton and Mr. F. E. Beddard, 

 we wish to give a few representative illustrations. Many 

 desert animals have an isabelline or sandy coloration that 

 renders them very inconspicuous ; the fennec fox and the 

 gerbille, the sand-grouse and the horned viper are good 

 examples. Green snakes are difficult to detect on the 

 trees and the common shore-crab, whose colour is variable, 

 often harmonizes to a nicety with the backgroimd of the 

 rock-pool. In many birds and mammals, as Thayer has 

 well shown, a very perfect garment of invisibihty is attained 

 in a very simple way — ^by having the under surface of the 

 body rather lighter than the upper surface. 



The protectiveness is heightened when the animal is 

 like something else, not in colour merely, but in form ; 

 and there is no better example than the Javanese butterfly 



