68 DEVELOPMENT AND HEREDITY. 



plete development, muscular action has begun in the 

 rhythmic pulsation of the heart. Evidently, the ner- 

 vous forces are active before the nervous structures 

 appear. There is no moment in the embryonic life 

 of an animal when we can say, " now the nervous 

 system assumes control over the life processes." The 

 nervous activity does not appear suddenly, nor sud- 

 denly concentrate itself in the nervous structures. 

 This nervous activity is something co-extensive with 

 the life of the animal. The nervous system is a 

 mechanism which has developed gradually — both in 

 the individual and in the species, — as the increasing 

 bulk and complexity of organisation made it neces- 

 sary for a more perfect action of the nervous force. 

 We must not allow ourselves to be blinded by a ter- 

 minology which is, unfortunately, inexact ; but we 

 must always bear in mind that it is not the visible 

 material substance of the nervous system which con- 

 trols the organism, but it is the forces which act 

 through the material substance, building up the po- 

 tential energy of the nervous system, and controlling 

 anddetermining its growth, as well as the growth of 

 the rest of the body. 



When we turn from the animal to the vegetable 

 kingdom, we find that the general rule holds good, 

 and that plants display certain forms of nervous ac- 

 tivity. All plants, under certain circumstances, show 



