RESPONSIVE LIFE OF ORGANISMS 



459 



The development of the cerebrum as an organ of memory 

 is the last and greatest step in the development of the sen- 

 sory mechanism in vertebrates. It makes education possi- 

 ble. Modes of action may be altered in the lifetime of the 

 individual, and new modes of action may be tried and if 

 found well approved by results, may, by repetition, be 

 made habitual. And it should be clearly apprehended that 

 the new control center does not replace the old. The in- 

 dividual has his lower reflex apparatus and correlation cen- 

 ters in charge of the ordinary operations of his body that are 

 necessary to keep life going, and, superadded thereto, he has 

 the new part by means of which he may continually be im- 

 proving his adaptation to environment. 



These then are the steps we have tried to trace in the per- 

 fecting of the nervous system as a seat of the sentient life : 



i) The differentiation among the cells of the primitive 

 metazoan of a) receptors (sense organs) ; b) contracting cells 

 (muscles), and c) communicating cells (nerves) connecting 

 the other two sorts. 



2) The withdrawal of the nerve cells from the surface of 

 the body to protected situations in the interior, and their 

 association together to form ganglia. 



3) The differentiation of neurones, bearing axones and 

 dendrites, and the development of polarity in them. 



4) The differentiation of neurones into two complemen- 

 tal sorts, and the association of these in pairs, forming reflex 

 arcs, each pair joining sense organ to muscle or gland. 



5) The development of intercalary cells in the nerve 

 centers, serving by their own intercommunicating processes 

 to bind together the reflex circuits of the body. 



6) The development of specialized sense organs, in con- 

 nection with some of the ganglia at the front end of the 

 body. 



