432 THE RESLXTS OF EVOLUTION 



tent among the flying squirrels, flying lemurs and certain 

 species of lizards. Here again the wings and planes are 

 adaptations with the same general function though the 

 organs themselves are structurally quite different. 



On the high seas many vertebrates, snails, crabs, 

 shrimps, jelly-fishes and the young of many other equally 

 distantly related species are transparent as glass, a con- 

 vergent adaptation, enabling such animals to escape the 

 notice of many sharp-sighted enemies. Numerous aquatic 

 organisms are provided with oil and fat which decreases 

 their specific gravity, or they are furnished with gas bags 

 or feather-like outgrowths which prevent their rapid sink- 

 ing. The heavy fore limbs of digging animals, the claws 

 of those which climb, the long legs and tendency to go 

 about on tiptoe common to many running species, the 

 more or less blind and colorless condition of cave animals, 

 the phosphorescent organs of numerous deep sea organ- 

 isms and many other examples known to the observant 

 student illustrate convergent adaptation. 



Convergent Adaptations among Plants. — In the 

 plant kingdom no less than among animals there are many 

 examples of convergent evolution. Numerous species of 

 widely different relationships have evolved the climbing 

 habit, in the struggle for existence. By means of ten- 

 drils, holdfasts, or by twining about some support they are 

 adapted to scale a wall or forest tree and expose their 

 leaves to the light. In the most extreme case, certain 

 palms, commonly known as rattans, form great festoons 

 in tropical forests, and not infrequently attain a length 

 of 600 to 900 feet, the greatest known length of any 

 organism. 



In the life of plants of arid districts it is of the utmost 

 importance that the water absorbed by the roots be con- 

 served as far as possible. To this end the leaf surface is 

 greatly reduced, thus preventing rapid evaporation; and 

 various species have evolved water storage tissue, giving 

 the leaves or stems a thickened fleshy appearance. These 



