2SO BEHAVIOR OF THE LOWER ORGANISMS 



the conduct of almost any of the lower animals for ten minutes can 

 scarcely fail to see the method exemplified. If he were watching a chick 

 pecking at a variety of objects and giving signs of disgust when it had 

 seized a nauseous substance, he would doubtless regard the process as 

 one of trial and error, whatever name he might apply to it. A study of 

 the conduct of much lower organisms would disclose many cases almost 

 equally evident. The lives of most insects, crustaceans, worms, and 

 hosts of lower invertebrate forms, including even the Protozoa, show an 

 amount of busy exploration that in many cases far exceeds that made 

 by any higher animal. Throughout the animal kingdom there is obedi- 

 ence to the Pauline injunction, ' Prove all things, hold fast to that which 

 is good ' " {I.e., p. io8). 



The well-known behavior of hermit crabs in finding suitable shells 

 in which to live and in changing shells which have become unsuitable 

 shows a systematic application of the method of trial extending to the 

 details of the behavior. This is well brought out in the excellent analysis 

 of this behavior given by Bohn (1903). 



Behavior of higher animals based on the selection of the results of 

 varied movements — the "method of trial and error" — plays, as is 

 well known, a large part in recent discussions of that subject. The 

 work of Thorndike (i8g8) on behavior in the cat, and the books of 

 Lloyd Morgan (1900), in which this matter is dealt with, are, of course, 

 well known, and require no discussion on our part. The fact that be- 

 havior of this character plays a large part in higher, as well as in lower, 

 organisms, is of the greatest interest, as showing that this method is 

 one of fundamental and general importance. But with the details in 

 higher animals we are not here concerned. 



3. MODIFIABILITY OF BEHAVIOR AND ITS DEPENDENCE ON PHYSIO- 

 LOGICAL States 



In the section preceding the present one we have described many 

 cases of behavior in the lower invertebrates in which the animal, under 

 the action of constant external conditions, passes from one form of be- 

 havior to another. All such cases are illustrations of the fact that be- 

 havior depends upon internal, physiological conditions, as well as upon 

 external stimuli. Since under the same external conditions the action 

 changes, the animal must itself have changed, otherwise it could not now 

 behave differently from before. It is clear that the continuance of a 

 stimulus, or the performance of a certain action, may change the physio- 

 logical state of the animal so as to induce new reactions. 



In some cases the varied actions performed under stimulation have 



