22 NATURAL HISTORY OF VERTEBRATE ANIMALS 



slippery surface makes it a difficult object for an animal like a 

 snake to hold between its jaws and begin swallowing. Some- 

 times, instead of endeavoring to escape, the frog will crouch 

 close to the ground and remain motionless, thus tending to evade 

 capture by hiding. When diving into the water, it sometimes 

 circles about and comes up to one side of the observer, near the 

 bank, among the weeds. Or it may hide on the bottom by crawl- 

 ing under some object. The tree frog, Bf^la versicolor, which pos- 

 sesses such unusual powers of changing its color to blend with sur- 

 rounding objects (Fig. 8), is perhaps aided by this reaction in its 

 struggle for life. 



While it would be difficult to prove beyond a doubt that these 

 varied activities of the leopard frog and its relatives are " adap- 

 tive " in the sense that they are frequently of life and death 

 importance, such responses are similar to the forms of behavior 

 exhibited by many other animals which, like the frog, possess no 

 special means of defense. Adaptation in animals has no doubt 

 been over-emphasized at times in the history of Biology; but the 

 fact remains that living organisms, both animal and plant, exhibit 

 a degree of adjustment to the necessities of their existence which 

 has impressed the biologist ever since Aristotle said that the 

 essence of a living being was '' fitness." 



Enemies and Parasites. — Being entirely without weapons or 

 defensive armor, save the tough and slippery skin, frogs are preyed 

 upon by many other animals and can find safety only in hiding or 

 flight. Next to man, snakes are undoubtedly their greatest 

 enemies, for, despite their lack of limbs, the snakes that frequent 

 banks of ponds and water courses are very adept at capturing 

 frogs and swallowing them, usually head foremost. Shore-feeding 

 birds such as cranes and herons, the common crow, turtles, such 

 fish as bass and pickerel, skunks, and many other animals all prey 

 upon frogs. Were it not for their rapid multiplication, these 

 Amphibia could hardly survive in the face of such destruction. 



Like most animals that have been carefully studied, the frog 

 is the " host " for a great number of parasites. Among those 

 likely to be met with even in a single frog are fluke worms of 

 the genus Pneumonwces, in the lungs; Clinostomum, encysted on 

 the inner surface of the body wall; Gorgoderina, in the urinary 

 bladder; and various other flukes within the intestine. In the 

 lungs may also be found a species of roundworm, Rhabdias, and 



