224 EVOLUTION AND GENETICS 



divided. Animal adaptations concern visual functions chiefly, 

 and in animals which have no eyes, the sensitiveness of the skin 

 to light rays. Burrowing animals may retain light sensitiveness, 

 as is true of the earthworm, but vertebrates like the mole tend to 

 lose their eyes. Such elaborate organs are obviously of no use in 

 darkness, and consequently they disappear or lose their functions 

 in cave-inhabiting animals as well as fossorial species. Sala- 



— ,..___^ manders and in- 



^v>^-^Z^~~^^nSv sects without eyes 



.-.,_^__^ ^lVIJ ^^^^ been recorded 



^^^'TT' 'JiiUi^^^ / / , ; /"7", ■", '""'^ frOm caves, and 



y'^'^'^ '^^ni ""^^^^^^nN""^^**^ some blind fishes 



(^,_-. — ^^^^ V from subterranean 



Fig. 132. — Blind salamander, Proteus anguinus, from waters (Fig. 132). 

 underground waters. A European species. (From Another charac- 



Lull, after Gadow.) , v.- i • j- -i 



ter which IS directly 



correlated with absence of light is reduction or loss of pigmenta- 

 tion. The presence of pigment in the skin acts as a protection 

 against the rays of the sun, as everyone has experienced through 

 sunburn followed by tanning. It is also of value in the develop- 

 ment of color and pattern for interrelations with other animals. 

 Where light does not penetrate, obviously neither use exists. 



Adaptation to Aridity. Animals. Water is essential to all life, 

 and is almost universally abundant. Only in desert regions is it 

 scanty, and animal adaptations to meet this lack are invariably cor- 

 related with adaptation to other conditions of desert life. Adapta- 

 tions for the conservation of water in animals are of three types: 

 (1) storage reservoirs, such as are found in the stomach of the 

 camel; these have already been mentioned ; (2) lack of the power 

 to perspire; (3) ability to absorb water through the skin. 



Plants. Plants require water as well as sunlight for photo- 

 synthesis, and since they cannot move about in search of it, but 

 must depend upon rainfall, their adaptation to lack of moisture is 

 not limited to desert species. If one month of the plant's life 

 must be passed without rain, it must be able to withstand this 

 lack of moisture or perish, no matter how great the abundance of 

 moisture at other times. Plants so adapted are known as xero- 

 phytes. They meet conditions of dryness in several ways, viz., 

 (1) by extensive root systems, which draw moisture from a large 

 volume of ground; (2) by reduction of leaf surface through small 



