376 COMPARATIVE ZOOLOGY OF CHORDATES 



the directions of the man at the wheel ; and it has been called 

 the head of the proprioceptive system. 



In bony fish, the cerebellum is enlarged to form the so- 

 called valvula which projects forwards beneath the roof of the 

 midbrain. In most amphibia and all higher vertebrates the 

 lateral-line system is lost except for the ear, and the cochlea 

 or organ of hearing is better developed. This affects the 

 cerebellum to some extent. In mammals two new features 

 arise, the superficial cerebellar cortex and the pons Varolii. 

 These develop in connexion with the cerebral cortex. 



Apart from the cerebellum, the correlation-centres are 

 mostly concerned with responses to the outside world. In the 

 fish there are correlation-centres of this kind in the forebrain 

 and the midbrain, but the most important are those which 

 become evolved above the evolutionary stage of the fish, and 

 which are situated in the sides of the between-brain (thalamus), 

 the floor of the end-brain (corpus striatum), and the roof of 

 the end-brain (cerebral cortex). 



It is characteristic of these higher centres of correlation 

 that they are more or less isolated from the primary sensory 

 centres ; in other words, the correlation-centres are not mono- 

 polised by any single sensory system. In much the same way, 

 if the government of a nation sat in the ordinary town-hall of 

 one of its cities, much of its business would be taken up or 

 influenced by local municipal matters, and it would be less 

 able to deal with business affecting not the city but the nation 

 as a whole. 



The thalamus is related by fibres to most of the sensory 

 centres, and it is among other things the centre where impulses 

 are analysed into pleasurable and painful. As such, it is of 

 great importance, for a negative reaction to danger and a 

 positive reaction to food and to a mate go far to ensure the 

 perpetuation of the species. Consequently the thalamus has 

 great survival value in evolution. 



The corpus striatum reaches a great development in birds, 

 in which it is responsible for the correlation of the many and 

 varied reactions and movements which form part of the 

 instinctive behaviour. Instinct in birds is highly developed, 



