THE ANALYSIS OF BEHAVIOUR 139 



muscle-nerve response, tropisms, taxis, reflexes, in animals sub- 

 jected to various kinds of operative interference, and ending 

 with the behaviour of the normal, intact higher animal. At the 

 beginning of such a series there is determinism, and at its end 

 there is indeterminism; but it would be rather difficult to suggest 

 any place where determined reaction became replaced by some 

 degree of chosen, deliberated behaviour. In such a series of 

 organic mechanisms we should be able to trace the beginning 

 and the gradual elaboration of a central nervous system. Nothing 

 of the kind is, of course, present in the compass-needle-magnet 

 system, and it is very doubtful if it can be recognised in the 

 typical plant organism. It is present in a very simple form (far 

 simpler than that which we have represented as present in the 

 earthworm) in the barnacle larva, and it becomes progressively 

 more complex as we ascend the vertebrate series. Somewhere, 

 then, in the ascending scale of living things indeterminism of 

 response is developed; we emphasise the word "response" so 

 that it may not appear that we exclude indeterminism of func- 

 tioning in the most general sense from the lower organisms. 



The Lower Brain Activities. 



We may next consider how the reactions exhibited by an 

 animal are modified when certain parts of the brain are removed, 

 or isolated from the other parts. Now, in such an animal as the 

 frog, both the cerebellum and the cerebral hemispheres are 

 undeveloped relatively to the medulla and mid-brain. The 

 former organ may be called rudimentary, and the cerebrum 

 consists of the two corpora striata, each with a rudimentary 

 cortex cerebri. It is an easy matter to remove the two cerebral 

 hemispheres (including the corpora striata), and the animal sur- 

 vives the operation. It is then in possession of a central nervous 

 system, including the parts developed from the three primary 

 brain vesicles, but lacking the fully developed cerebellum which 

 is present in most vertebrate animals higher than the reptiles. 



Now every action that can be performed by the intact frog 

 can equally well be performed by the decerebrate frog 'provided 

 that a suitable stimulus he applied. It can swim, leap, and crawl, 

 and it assumes, when motionless, the natural posture of a frog. 

 It can avoid obstacles when it is moving, and it is sensitive to 

 light. Its visceral, respiratory, and nutritive organs function 

 normally. It swallows food which is placed in its mouth. If 



