582 RAYMOND PEARL. 



is apparent!}' no upper limit) varies to some extent with the 

 physiolog-ical condition of the individual. Thus in some 

 specimens at certain times stimuli which would ordinarily 

 produce a rather strong negative reaction will call forth 

 uotliing but the positive reaction. This condition is only a 

 transitory one, and the reason for it seems to be a heightened 

 tonic condition of the animal. Specimens exhibiting this 

 relation to rather strong stimuli are always very active, and 

 move about with great rapidity, frequently raising the 

 anterior end of the body and waviug it about through the 

 water as they glide along. Persistent strong stimulation 

 of the organism rapidly changes the general physiological 

 condition. This is not more true of stimulation applied to 

 the head region than of strong mechanical stimulation of any 

 part of the body. The animal becomes " stirred up" gene- 

 rally, moves about with increased rapidity, its sensitiveness 

 to stimuli becomes diminished, and it will give only the 

 negative response to stimulation of the anterior end. This 

 change in the physiological condition of the animal as a 

 result of continued stimulation of any sort, as in a series of 

 experiments, is a matter of great practical importance in 

 connection with reaction work. One may get totally differ- 

 ent appearances from an individual which has been " stirred 

 up " from what are seen in the case of one "svhich is in the 

 normal condition. This is only one of a number of factors 

 which must be taken into account in woi-k on the reactions 

 and behaviour of an organism if one is to obtain trustworthy 

 results. It is almost an absolute necessity that one should 

 become familiar, or perhaps better intimate, with an organism, 

 so that he knows it in something the same way that lie 

 knows a person, before he can hope to get at even an approxi- 

 mation of the truth regarding its behaviour. 



/3. Reactions to Weak Stimuli. — The positive reac- 

 tion is the characteristic reaction given to all weak stinnili. 

 It is an orienting reaction in the sense that it brings the 

 anterior end of the animal in a position such that it points 

 np]iroxim!\toly towards the source of the stimulus. On 



