32 BIOLOGY 



nervous system. From these, forms are reached in 

 which there is a highly complex system of superficial 

 threads which ramify throughout the body, with all of 

 its parts equally important. But soon there is a 

 specialisation of some tract or other, and all the rest 

 may be said to be branches of it, and the worms afford 

 an excellent example of this division into a main or 

 central part and a subsidiary or peripheral portion. 

 The next complication is the gradual assumption by 

 some portion, usually the anterior, of this nervous system, 

 of a higher complexity of structure and function, and 

 we get the first hints of a brain. 



When we consider this more carefully, it is probable 

 that it is the increasing complexity of the peripheral 

 portion that is the exciting cause of the formation of a 

 brain, with all its complex arrangement, by means of 

 which the stimuli received from the external world are 

 received, duly recorded, and acted upon. It is difficult 

 to explain exactly the complexity of the various func- 

 tions of the brain without entering into minute details 

 which would benefit us little ; but this much may be 

 said : that a complicated system of nerve strands must 

 be acted upon before the simplest external or internal 

 stimulus can be received and acted upon by the brain. 



The mere development of a brain is not the end of it 

 all. For as the higher forms are evolved, a finer series 

 of organs are also formed, to each of which the duty of 

 looking after some particular kind of stimulus is dele- 

 gated. The eye, for instance, is set aside for the recep- 

 tion of light-stimuli, the ear for the vibration-stimuli, and 

 so on. Now as each of these special organs gradually 

 developed, there was a corresponding increase in brain- 



