38 THE BIOLOGY OF THE FROG Chai>. 



the green and golden colors come out to a much greater 

 extent, and the black pigment cells become less conspicuous. 

 There is Httle doubt that power of color change in Rana 

 pipiens is of service to the animal as a means of conceal- 

 ment. The frog is less conspicuous in a dark environment, 

 when its skin assumes a darker hue, and when in the grass 

 or weeds its green coloration serves the same purpose. The 

 mechanism of color changes, and the various stimuli by 

 means of which they are set up, will be treated of in the 

 description of the skin. 



Enemies. — As frogs are among the most defenseless of 

 animals, they fall an easy prey to a variety of carnivorous 

 creatures, who devour them in great numbers. First of 

 these enemies in order of destructiveness is doubtless to be 

 counted man, who, on account of his fondness for frogs' 

 legs, to say nothing of his scientific curiosity, has almost 

 exterminated some species in many localities. It is in the 

 breeding period in the early spring that the destruction of 

 frogs is greatest, since the animals then appear most abun- 

 dantly and are most easily caught. Water rats and skunks 

 catch many frogs, the latter in Europe, according to Fischer- 

 Sigwart, hunting out the frogs from the hollows in which 

 they often congregate during the winter. There are a num- 

 ber of birds which prey upon frogs, such as cranes, herons, 

 and crows ; but their greatest enemies, next to man, are the 

 various species of snakes, of which, according to Fischer- 

 Sigwart, they have an intense instinctive fear. When in the 

 water they may also fall a prey to the larger species of 

 turtles. 



In Europe several fishes, such as the larger herring and 

 trgut, prey. upon frogs; and smaller fishes are very destruc- 

 tive to the tadpoles. 



To a certain extent frogs are preyed upon by other mem- 



