140 THE MECHANISM OF LIFE 



it is turned over on to its back it can regain its proper 

 attitude. 



But it does none of those things spontaneously, and if it is not 

 stimulated it makes none, or very few, movements of its own 

 accord, and it nearly always remains in exactly the same position 

 if it is left strictly alone. When it is stimulated it responds in 

 an almost invariable way, and the effect of any particular 

 stimulus can be predicted. What is wanting in its reactions 

 and movements is the evidence of spontaneity: there is no 

 " intrinsic stimulus," and nothing that indicates the possession 

 of volition. It is " a machine, and nothing more, while the frog 

 possessing its cerebral hemispheres is a machine governed and 

 checked by a dominant volition." 



Its responses to external stimuli indicate that it possesses the 

 power of completely co-ordinating its movements. That power, 

 we shall see, is dependent in higher vertebrates on the presence 

 of the cerebellum, and on the full connections of this organ with 

 the rest of the nervous system. But in the frog the cerebellum 

 is rudimentary, and we must therefore conclude that the func- 

 tions that it performs in the higher vertebrates are carried out 

 by the mid-brain. Therefore we cannot say that co-ordination 

 is the work of the cerebellum exclusively in all vertebrates. It 

 can be effected by other parts of the brain. 



Now remove the cerebral hemispheres — that is, the corpora 

 striata and pallia — from a fish, and the animal survives the 

 operation. But there is no absence, " not even the temporary 

 absence," of apparently spontaneous movements. It is there- 

 fore evident that spontaneity of behaviour, which depends, in 

 the frog, upon the presence of the cerebral hemispheres, can be 

 mediated, in the fish, by the mid and hind brain, just as co-ordi- 

 nated movements, which depend, in the bird, upon the presence 

 of the cerebellum, can be mediated by the mid-brain in the frog. 



The cerebral hemipheres are much more highly developed in 

 the birds than in the amphibia, but they can be removed and 

 the animal (the pigeon) can be kept alive for a considerable time 

 after the operation. The same general effects observed in the 

 frog can also be seen in the decerebrate bird. It either remains 

 quiet and impassive, or it moves about restlessly and without 

 any apparent purpose. Of itself it does not attempt to fly, but 

 if it is thrown into the air it will fly and avoid obstacles. It 

 does not spontaneously pick up corn, though it will do so if its 



