STIMULATION OF LIVING ORGANISMS 579 



THE GENERAL PHYSICO-CHEMICAL CONDITIONS OF 

 STIMULATION IN LIVING ORGANISMS 



By Professor RALPH S. LILLIB 



CLARK UNIVERSITY 



IT is customary to say that irritability, or the capability of responding 

 to stimuli, is an essential characteristic of living beings. Whether 

 this is true or not of the lowest organisms — certain bacteria or the 

 filterable viruses — there is no doubt that it is preeminently so of the 

 higher, and especially of those leading free and active lives, like most 

 animals. If this were not the case it is difficult to see how such organ- 

 isms could maintain themselves in their surroundings and continue to 

 behave as living beings — i. e., show their characteristic activities, grow, 

 and eventually reproduce themselves. At least it is clear that in order 

 to do this they must react to the changes continually taking place in 

 their particular environment in such a way as to favor their continued 

 existence in that environment; if, for instance, any animal failed to 

 respond to the presence of food — material that can serve it as source of 

 energy — by capturing and incorporating enough to replace its own nor- 

 mal loss of substance, quite obviously its life would soon come to an end. 

 And if it reacted in the same way to the poisonous or otherwise injurious 

 substances in its surroundings as to food, and incorporated both classes 

 of material indifferently, the same result would follow. Evidently there 

 is needed some power of active and selective response to the changing 

 conditions of the environment if the living organism is to continue to 

 live; it must preserve a certain equilibrium with its surroundings; the 

 materials and energy which it appropriates from those surroundings 

 must in the long run at least equal those which it inevitably loses to 

 them in the normal course of its vital processes. This is the physiologi- 

 cal interpretation of Spencer's dictum that all life involves a continued 

 adjustment between internal and external relations. The organism must 

 continually alter its activities in correspondence with altered conditions 

 in its surroundings, and in such a way as to preserve this adjustment — 

 avoiding conditions likely to disturb or destroy the vital equilibrium and 

 tending to place itself in those favorable to its continuance. Accordingly, 

 we may say that living organisms in general, and especially animals, ex- 

 hibit two broad classes of reactions, first those of a defensive or protective 

 kind, including avoiding reactions and inhibitions of various kinds, and 

 second, the more active group of what we may call self-seeking or 

 acquisitive reactions ; of these the chief are the reactions of food-seeking, 



