506 COLLEGE ZOOLOGY 



through the pupil, and the impulses which are carried through 

 the optic nerve to the brain give rise to sensations of sight. 



Behavior. - - The activities of the frog are such as to enable 

 it to exist within the confines of its habitat. The ordinary 

 movements are those employed in leaping, diving, crawling, 

 burrowing, and maintaining an upright position. These and 

 most of its other activities may be resolved into a series of reflex 

 acts, although they are commonly said to be instinctive. In- 

 stinct is " the faculty of acting in such a way as to produce cer- 

 tain ends, without foresight of the ends, and without previous 

 education in the performance." (James.) 



Some of the movements of the frog are due to internal causes, 

 but many of them are the responses to external stimuli. Frogs 

 are sensitive to light, and experiments have shown that both the 

 eyes and skin are stimulated by it. The reaction to light causes 

 the animal to orient its body so that it faces the source and is in 

 line with the direction of the rays. Nevertheless, frogs tend to 

 congregate in shady places. Frogs also seem to be stimulated 

 by contact (thigmotropism, p. 36), as shown by their tendency 

 to crawl under stones and into crevices. The desire for shade 

 may, however, have some influence upon this reaction. The 

 temperature modifies the responses both to light and to contact. 



Investigators who have studied the behavior of frogs have 

 come to the conclusion that they are very stupid animals. It 

 is possible to teach them certain things, and habits once formed 

 are not easily changed. For example, Yerkes found that a frog 

 could learn to follow a path in a labyrinth after about one hun- 

 dred trials. If we consider the power to learn by individual 

 experience as evidence of the presence of mind, then we must 

 attribute a primitive sort of mind to the frog. 



Development. - - Frogs lay their eggs in water in the early 

 spring. The male clasps the female firmly with his fore legs 

 just behind her fore legs. After the male has been carried about 

 by the female for several days, the eggs pass from the uterus out 

 of the cloaca and are fertilized by the spermatozoa of the male, 



