ii6 Journal of Comparative Neurology. 



involve imitation, hunting, fighting, rivalry, acquisitiveness, 

 and construction, combined in various ways ; theii special rules 

 are habits, discovered by accident, selected by intelligence and 

 propagated by tradition." This is the most scientific definition 

 we have yet found and may be employed as a basis for an exam- 

 ination of our common animals. The simplest observation 

 shows that play has its physiological condition m redundant 

 physical power or excitement. No animal plays when fa- 

 tigued or weak. There must be a supply of explosive 

 material in unstable equilibrium which produces a ner- 

 vous plethora. We may imagine that the constant nervous 

 overflow which constitutes the tone of a healthy system gradu- 

 ally increases until it becomes in itself an active irritant — a suf- 

 ficient cause for motion. It is one of the simplest law of physiol- 

 ogy that the discharge of superabundant accumulation is always 

 attended with more or less pleasure, the amount depending on 

 the extent to which the discharge may be attended with impor- 

 tant results to the individuals or species. The discharge of 

 nervous and muscular power is no exception. 



The simplest forms of play are due almost exclusively to 

 exuberance of physical vitality. Thus when my three-months' 

 kitten rises from its sleep, yawns and suddenly springs away, 

 rushes across the lawn and climbs half way up the tree, and 

 then, as if remembering the dignity of cat-hood climbs down 

 and sedately returns to its nap, it may be be presumed that there 

 was no consciousness of purpose or motive or definite objective 

 point, but a mere impulse to activity which received its uncon- 

 scious direction from the circumstances of its environment and 

 sensations in its muscles. 



But such blind, spasmodic, impulsive exercise is scarcely 

 entitled to be called play. At most it seems to give us the hint 

 as to the primary physical impulse back of play. What may be 

 called the psychical condition is the absence of other forms of 

 mental activity. If attention is otherwise occupied the instinct 

 to play is inhibited. Strong incentive to hunt, like hunger, or 

 great alertness, as when the attention is riveted on a hole and 

 the slightly twitching tip of the tail is the only evidence of ac- 

 tivity, are incompatible with the play instinct. These are the 

 two fundamental laws, positive and negative, at the basis of the 



