CHAPTER LXVII 



PROTECTIVE PIGMENTS AND APPEARANCES 



400. Pigments and light. Animals that live at great depths 

 of the sea, and those that live in caves, situations in which 

 there is little or no exposure to light, do not generally show 

 much pigment in the skin. 

 This fact may be interpreted 

 in two ways : 



1. Where there is no dan- 

 ger of being injured by light, 

 the species will be able to main- 

 tain itself without acquiring 

 the pigment-forming habit. 



2. Where there is no light 

 stimulation, pigment will not 

 be formed. 



In the human race the dark 

 pigment of the skin is un- 

 doubtedly a protection against 

 the light, as shown by the 

 relative sensitiveness of light- 

 skinned races and dark- 

 skinned races to the influence of the tropical sun. It is also 

 shown by the behavior of the skin of a person who has been 

 tanned and the behavior of the skin of the same person before 

 the tan has formed. A person who does not get tanned is likely 

 to be sunburned with every exposure to strong sunlight. On the 

 other hand, in a person who is dark-skinned, or who has be- 

 come tanned, the pigment acts as a screen, cutting off the rays 

 that are injurious to the protoplasm. 



35 1 



FIG. 167. Katydids 



Microcentrum retinervis (above) ; Cyrtophyl- 

 lus concavus (below). These insects match 

 the color of the foliage upon which they 

 feed ; in some species the resemblance to a 

 green leaf is even more striking than in the 

 two shown here 



