206 DOCTRINE OF EVOLUTION 



light of day and digs its burrow and seeks its food by 

 wonderfully coordinated activities of its muscles and 

 other parts, which are controlled by a double chain of 

 ganglia along its ventral side, connected with a similar 

 pair of grouped nerve-cells above the anterior part of 

 the digestive tract. The ganglia of each segment 

 exercise immediate supervision over the structures of 

 their respective territory, while they pass on impulses 

 to other ganglia so that movements involving many 

 segments can be properly adjusted. Everything an 

 earthworm does is controlled by the cells grouped in 

 these ganglia, or scattered along the intervening con- 

 necting cords. We speak of its acts as instinctive, 

 employing a term which seems to indicate a different 

 kind of operation carried on by the nervous system, 

 but a moment's thought will show that an instinctive 

 act is simply a complex group of reflex acts. The 

 physical basis and ultimate unit is a cell, and the func- 

 tional unit is likewise a cell act ; therefore the seeming 

 difference proves to be one merely of degree and not of 

 kind. The greater complexity of the worm's nervous 

 system as compared with that of Hydra gives to the 

 whole mechanism a plasticity that diverts the attention 

 from the mechanical nature of the entire instinctive 

 act and of its basic cell elements. 



The instinct, like the elementary reflex, is determined 

 by heredity. Because a certain configuration of the 

 cells and fibers making up a nervous system is inherited 

 as well as the characters of the constituent elements 

 themselves, a worm or an insect is enabled to act as it 

 does. A butterfly does not have to learn how to fly, 

 for it flies instinctively. When it emerges from its 



