ADAPTATION 333 



Nerves are distributed principally to the upper layer. The 

 electroplax, which represents the electric unit in the organ, seems 

 to be modified striated muscle tissue. The discharge is brought 

 about by nervous stimulation, the quantity of the discharge 

 varying with the size of the organ and its state of fatigue. If a 

 small Torpedo is grasped by the thumb and fingers in the upper 

 and lower surface of the body, respectively, a slight pressure 

 brings forth a distinct shock, which produces a tingling sensation 

 in the hand. Electric organs are relatively rare among animals. 

 If the electroplax be regarded as a modified muscle fiber, the 

 relationship between the nervous stimulation and the electrical 

 discharge can be understood when it is remembered that the 

 contraction of a muscle occurs through nervous stimulation and 

 that the contraction is accompanied by minute electrical dis- 

 charges. In the case of the electroplax, the conditions seem to 

 be reversed to the extent that the major result of nervous stimula- 

 tion is the production of an electric current rather than motion. 

 The outstanding features of the electric organ can thus be 

 explained as the result of a change in the functional response of 

 what in other animals is a contractile tissue. 



Protective Coloration. — A general harmony in the coloration 

 of an animal and its surroundings is a common condition in many 

 animals. The coloration of the animal is rarely an exact duplica- 

 tion of a part of its natural surroundings; often it is merely a 

 neutral tint which blends with the surroundings when the animal 

 is at rest. Thus the fur of the rabbit, Sylvilagus floridanus, is a 

 shade of gray which makes the animal inconspicuous against 

 almost any natural background. This same color tone is often 

 found in many small and some large fur-bearing animals. In 

 other cases there is a closer correspondence between the coloration 

 of the animal and its surroundings. Among amphibians for 

 example, tree frogs, such as Hyla andersonii, which are found in 

 leafy trees, are bright green. Hyla arenicolor, another species, is 

 found on rocks and is gray. Rana pipie7is, the grass frog, is 

 greenish in color with spots and stripes which adapt it to a 

 meadow environment. The value of interpretation of coloration 

 comes into question when forms with conspicuous color patterns 

 are found living side by side with inconspicuous forms. Thus 

 Amby stoma maculatum, a common salamander which has a black 

 body with brilliant yellow spots, lives in the same habitat with 



