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 physical 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 
