II HABITS AND NATURAL HISTORY OF THE FROG 29 



fore, it rises in the water. When at the surface the frog 

 usually lies quiet, hanging obliquely with the hind legs in a 

 state of moderate extension. The fore legs generally are 

 held out from the body. In such a position the frog may 

 rest for a long time without performing any other move- 

 ments besides those involved in respiration. The extended, 

 sprawled-out attitude of the frog when resting at the sur- 

 face contrasts markedly with its resting position on land, 

 when its hind legs are closely doubled up and already set 

 for a spring. One probable reason for the extension of the 

 hind legs is that there is nothing to support them from 

 below, and they would naturally hang down, when relaxed, 

 from their own weight. However this may be, the extended 

 condition of the hind limbs is of service in enabling the 

 animal to suddenly draw itself downward whenever danger 

 threatens from above. 



Diving. If a frog is approached when it is resting at the 

 surface of the water, it will dive downward with great celerity 

 and make several strokes, carrying it some distance away 

 from its resting place. The action is performed so quickly 

 that it is not easy at first to see how it is accomplished. At 

 one moment the frog is resting in perfect quiet and at the 

 next instant we perceive him making vigorous kicks and 

 rapidly swimming away. By experimenting with frogs kept 

 in a glass dish and concentrating our attention on one 

 feature of their behavior at a time, we may gain an idea 

 of the way this feat is accomplished. To swim downward 

 through the water the animal has to reverse its position, as 

 an extension of the hind legs in its normal resting attitude 

 would tend to throw it out of the water. The first move- 

 ment is that of withdrawal from the surface, which is accom- 

 plished by suddenly bringing the hind legs forward, thus 

 giving the body a backward impulse. This brings the hind 



