6 



association and differentiation in nerve-routes make ac- 

 quirement and development possible. 



I said tliat the fundamental plan of nerve-organisation 

 is exceedingly simple. The ideal plan, which is that simple 

 form to be found in lowly organisms, is as follows. An 

 ingoing nerve consisting of nerve-filaments conveys the 

 sense-impression, and hence is called a sensory nerve. It 

 ends in cells collected together and called the sensory 

 centre. These are connected by nerve-filaments with 

 another group of cells called a motor centi'e, and from this 

 centre the outgoing impression is conveyed along a motor 

 nerve. The motor and sensory centres are usually, how- 

 ever, associated together, and are also connected with 

 other motor and sensory centres. Consequently an im- 

 pression is conveyed along the sensory nerve to a nerve 

 centre which is a point of meeting, or a junction-station as 

 it were, where it is brought into relation with other 

 impressions, shunted on to another line it may be, com- 

 bined with other impressions or separated into its various 

 components, until it eventually makes its exit in some way 

 or other along a motor track. 



Just as efficient railway communication depends on the 

 development of junctions, so all advance depends on the 

 progressive association and complexity of nerve-centres. 



Thus, the caterpillar has many pairs of nerve-centres, 

 each pair corresponding to a segment of the body. But 

 when it becomes a highly organised insect, the nerve- 

 centres collect together to form fewer but more complex 

 ganglia. Similarly in the ascending series of the crab and 

 lobster tribe, we find an increasing coalescence of ganglia 

 or nerve-centres. The higher and more complex the 

 functions, the more intimately associated and complex are 

 the ganglia. 



The development in the animal series of complex co- 

 ordinated movement as well as of intelligence, depends on 

 the development of the organs of sense. The principal 

 collection of ganglia, called the brain, is usually in the 

 head of the animal ; and its development is directly pro- 

 portional to the development of the organs of sense. I 

 shall endeavour to show why the sense-organs are situated 

 near one another, and hence why their associated ganglia — 

 the brain — is located in the head. 



By the repeated stimulation of contact with food, the 

 reason is evident why the sense of touch should be devel- 



