Factors in Evolution 27 



Cumulative Effects of Stimuli. It has often been 

 demonstrated that the effects of stimuli may be 

 cumulative, and that when a stimulus is repeated at 

 short intervals the response to this may be very 

 different in the later cases of stimulation. Thus 

 Jennings, in his important studies on the reactions 

 of the Infusoria, has shown that they may become 

 habituated to a stimulus, and fail to respond to it 

 again after it has been repeated several times. It 

 would appear that the " physiological state " of the 

 cell has changed as the result of the stimulus, and 

 in a very suggestive recent address by Prof. F. 

 Darwin, he brings forward the theory that this 

 permanence of the effects of stimulation upon the 

 protoplasm, or " memory " as he boldly puts it, is 

 perhaps the most potent of all causes in determining 

 the course of evolution in the development of an 

 organism. 



Experiments with Slime-molds. Where proto- 

 plasm occurs in large masses, as it does in those 

 remarkable organisms the slime-molds, its reaction 

 to various stimuli is easily demonstrated. Exposed 

 to strong light, the slimy mass or " plasmodium " 

 will seek shelter in the crevices of the rotten log on 

 which it is growing or will hide under the masses of 

 dead leaves or tan bark which may serve it for 

 food. The movements will be accelerated by an 

 increase of temperature ; withdrawal of moisture 

 will cause a contraction or even make it assume a 

 quiescent stage; and in short the sensitive mass of 



