II HABITS AND NATURAL HISTORY OF THE FROG 37 



thicker than it is elsewhere. Since these organs appear 

 during the breeding season, it is probable that they have 

 some function in relation to reproduction. If they do not 

 directly serve to enable the male to retain his hold of the 

 female, they may act as stimuli, causing him to clasp more 

 tightly when he feels the female slipping from his grasp. 



Color Changes. — One of the most remarkable adaptations 

 of many kinds of frogs for concealment from their enemies, 

 is the power of changing their color in harmony with their 

 surroundings. The tree frogs possess this property in the 

 highest degree. When these animals are among the green 

 leaves of a tree, they assume a bright green color. When 

 on the bark, their skin turns to a gray or brown. In both 

 cases the color of the frog closely resembles that of the 

 surroundings and serves to make its possessor difficult to 

 distinguish. The value of such a power as a means of 

 protection from enemies is obvious. No frog, however 

 remarkable may be the changes in color it may undergo, is 

 able to assume all shades and hues. Frogs possess the 

 property of adapting themselves only to the predominant 

 colors of their environment, which are green, the color of 

 vegetation, and some shade of gray or brown, the usual 

 color of the soil and the bark of trees. They cannot turn 

 red or blue or violet, and, in fact, the power to do so would 

 be of little value to them if they possessed it. 



Rana pipiens, like most of the members of its genus, 

 possesses a much less range of color variations than the 

 tree frogs ; nevertheless it can change its color to quite a 

 marked degree. If in a dark environment, its skin becomes 

 much darker; the black spots contain so much pigment 

 that they remain unchanged under all conditions, but the 

 lighter regions between them are subject to marked changes. 

 Exposure to bright light gives the skin a much lighter color, 



