18 THE RIDDLE OF MIGRATION 



multiplication and the more numerous they become, 

 the more intimately is one sense put in touch with 

 another and the more elaborate become the 

 responses. 



But another and better mode of association exists 

 in what is termed a correlation centre, best repre- 

 sented in the brain of fishes (and all higher verte- 

 brates) in that part of the brain known as the 

 cerebellum. This is really the unit that regulates 

 bodily activity. It is not the seat of any individual 

 sense but receives messages from all the senses. 

 They are here coordinated. In the fish's brain we 

 find exactly what we should expect, a comparatively 

 large cerebellum, for the fish is a relatively active 

 animal. The same is true of birds. In the sluggish 

 reptiles it is small. 



The most important part of the primitive brain 

 was the paired olfactory lobe constituting most of 

 the anterior end. It is very large in fish but tends 

 to become subordinated in the higher vertebrates. 

 One reason for this is that other brain tracts keep 

 encroaching on its space so that we slowly get it 

 replaced to varying extents by other units. There 

 has also been a tendency to build over it, i.e. non- 

 olfactory tracts have not only invaded the precinct 

 of the olfactory section and actually reduced it, but 

 they have been superimposing new material simul- 



