122 ETGHT-LEGGED FRIENDS. 



webs by whose aid, though wingless, they float balloon- 

 wise through the air ; that others employ them to line 

 the sides of their underground tunnels, and to make the 

 basis of their marvellously ingenious earthen trap-doors ; 

 that yet others have learnt how to adapt these same 

 organs to a subaquatic existence, and to fill cocoons with 

 air, like miniature diving bells ; while others, again, have 

 taught themselves to construct webs thick enough to 

 catch and hold even creatures so superior to themselves 

 in the scale of being as humming-birds and sunbirds. 

 This extraordinary variety in the utilization of a single 

 organ teaches once more the same lesson which is 

 impressed upon us elsewhere by so many other forms of 

 organic evolution : whatever enables an animal or plaut 

 to gain an advantage over others in the struggle for life, 

 no matter in what way, is sure to survive, and to be 

 turned in time to every conceivable use of which its 

 structure is capable, in the infinite whirligig of ever- 

 varying nature. 



