134 Journal of Comparative Neurology and Psychology. 



I. Reflex, if, for example, the reaction series 

 read i-i-i-i-i-i or 2-2-2-2-2-2 or 3-3-3- 

 3-3-3 etc. 

 II. Instinctive if the reaction series read i, 2— 1, 

 2—1, 2—1, 2 etc. In this series of numerals 

 differences in weight indicate slight variations 

 in the emphasis of the various units of the ac- 

 tion. 

 III. Voluntary if the reaction series read i, 2, 3 — 

 2, 3-3-2-1, 3 etc. 

 Leaving difference in complexity out of consideration for 

 the moment, the reflex act is uniform, the instinctive variable, 

 the voluntary unique. 



'Having now defined our reaction types, we may note that 

 comparative studies show that animals differ with respect to 

 their activities in two fundamentally import mt directions : first, 

 in the number and variety of the reactions which they are -ca- 

 pable of giving in response to a particular stimulus-complex 

 (situation); and second, in the rapidity with which they are able 

 to narrow down the trial and error series of reactions to those 

 acts which are appropriate to the situation. From one point of 

 view this is learning, from another it is the mechanizing of ac- 

 tivity. The former of these differences may be known as the 

 scope of reaction; the latter, as ability to profit by experience. 

 Organisms in markedly differing degrees, according to their 

 complexity of form and environmental relations, follow the in- 

 junction "prove all things ; hold fast that which is good." 



Of chief interest in this connection is the recently discovered 

 fact that even the simplest organisms are not exceptions 

 to this rule, for Jennings has noticed that Paramecium, 

 Vorticella, Stentor and many other unicellular forms exhibit 

 the trial and error method of reaction ; and more recently 

 Holmes^ has described the same type of reaction in the 

 earthworm and other invertebrates. These discoveries prove 

 the falsity of the prevalent notion that simple organisms exhibit 

 single simple stereotyped reactions which are suitable in 

 the long run, but show no signs of adaptation or even of 



' The selection of Random Movements as a Factor in Photota.vis. This 

 Journal, 15, loo. 



