112 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



tialities of action, and will exhibit the corresponding 

 acts, under the influence of the appropriate stimuli. 

 A large proportion of these stimuli come from without 

 through the organs of the senses. The greater or less 

 readiness of each sense organ to receive impulses, of 

 the nerves to transmit them, and of the ganglia to 

 give rise to combined impulses, is dependent at any 

 moment upon the phj^sical condition of these parts ; and 

 this, again, is largely modified by the amount and the 

 condition of the blood supplied. On the other hand, a 

 certain number of these stimuli are doubtless originated 

 by changes within the various organs which compose the 

 body, including the nerve centres themselves. 



When an action arises from conditions developed in 

 the interior of an animal's body, inasmuch as we cannot 

 perceive the antecedent phenomena, we call such an 

 action "spontaneous;" or, when in ourselves we are 

 aware that it is accompanied by the idea of the action, 

 and the desire to perform it, we term the act " volun- 

 tary." But, by the use of this language, no rational 

 person intends to express the belief that such acts are 

 uncaused or cause themselves. " Self- causation " is a 

 contradiction in terms ; and the notion that any pheno- 

 menon comes into existence without a cause, is equivalent 

 to a belief in chance, which one may hope is, by this 

 time, finally exploded. 



In the crayfish, at any rate, there is not the slightest 

 reason to doubt that every action has its definite physical 



