344 



THE BIOLOGY OF THE FROG chap. 



not commonly call instinctive, but which are as well adapted 

 to the end achieved as the diving of a frog upon the ap- 

 proach of an enemy, or its burrowing into the mud in cold 

 weather. All of these actions are purposive responses to 

 stimuli. Those of the stomach we call reflex acts, while the 

 diving and burrowing into the mud are usually called in- 

 stincts. The chief distinction between the two is that the 

 one involves the action merely of a part, while in the other 

 there is a response by the organism as a whole. There are 

 so many intermediate types of reaction, however, that it 

 is no easy matter to decide how some of them should be 

 classed. If a frog withdraws its foot when its toe is stimu- 

 lated, we call the act reflex, but how shall we designate the 

 act of bringing the foot forward to wipe away a drop of acid 

 from the side of the body? As we have seen, the latter act 

 may be performed by a frog whose spinal cord is cut across 

 near the brain ; if, therefore, we call the action reflex, what 

 shall we say of the struggles of a frog when, after being 

 picked up in the fingers, it uses both hind legs to push 

 against the hand, and at the same time inflates the lungs 

 with air, causing the body to swell? These struggles to es- 

 cape will take place in a frog which has lost the greater part 

 of its brain, but nevertheless, they would, I think, generally 

 be regarded as instinctive actions. The use of the hind 

 limbs and the swelling of the body may be regarded as two 

 complex reflexes excited by the same cause. The frog is so 

 organized as to respond to seizure by two methods which 

 cooperate to effect its escape, both of which are reflexly 

 brought into play. 



Many of the more complex instincts of the frog may be 

 resolved into a series of reflex acts. We have seen that the 

 tendency of the male frog to clasp the female during the 

 breeding season depends upon the reflex irritability of 



