14 THE USES AND ORIGIN 



leading to the union of male and female, the begetting of 

 young, and the consequent perpetuation of the species. 



Thus it may be dimly gathered how complex the relation 

 of the animal organism to its environment soon becomes, 

 and also what an amount of interdependence is estabP.shed 

 between the actions of the several parts or organs of the 

 animal economy. The contrast between the animal and 

 the vegetal organism in both these respects becomes most 

 marked. 



It is during the establishment of the complex relations 

 above indicated between an animal and its environment, 

 and between the several parts or organs of an animal, that 

 nervous tissues first take origin, develop, and subsequently 

 increase in complexity. How and why this should be may 

 become a little more plain after a brief consideration of 

 the nature of simple nervous functions and structures, 

 and after some reference to the manner in which these 

 increase in complexity, not only in the individual but (by 

 virtue of the principles of heredity and ' natural selection ') 

 during the life of that succession of individuals consti- 

 tuting the race or * species ' to which the organism 

 belongs. 



From what has been already said it will be seen that 

 the preliminary conditions necessary for the initiation of 

 a Nervous System are, first, the existence of a living sub- 

 stance whose excitability is high ; and, secondly, the pos- 

 session by such substance of a well-marked contractile 

 power. This statement carries with it the implication 

 that the living matter in which a nervous tissue is to 

 develop must not, in the first place, subdivide itself very 

 minutely into separate units ; or, at all events, that it 

 must not become differentiated into cells with fully de- 

 veloped cell-walls. Much of the substance of the organism. 



