The Beginnings of Intelligence 125 



A very general change of behavior in organ- 

 isms consists in the habituation to any stimulus which 

 is repeated at sufficiently close intervals so that the 

 organism no longer responds to it. This is shown 

 even among the protozoa. A Stentor or a Loxo- 

 phyllum subjected to a light mechanical stimulus at 

 short intervals soon fails to respond as at first, but 

 the duration of the modification so produced is very 

 short; in Loxophyllum it probably does not extend 

 over two or three seconds. Similar effects of re- 

 peated stimulation but of longer duration have been 

 observed in Hydra, several species of sea-anemones, 

 planarians, annelids and various other lower inver- 

 tebrates. As a rule failure to respond may occur 

 more quickly and the effects of the stimulus remain 

 longer as we pass up the scale of animal life. 



Occasionally the reverse phenomenon occurs 

 when the response to a given stimulus is increased 

 instead of diminished with repeated applications — 

 a result which suggests the effect of the summation 

 of stimuli. At times, as Bohn found in Cerianthus, 

 there is an initial increase of responsiveness fol- 

 lowed by a' dulling of sensitivity. Bohn has at- 

 tempted to subsume the effects of repeated stimula- 

 tion under a general "law" to the effect that stimu- 

 lation always produces at first increase of sensitivity 

 to be followed later by a decrease. Sometimes, as 

 Bohn claims, the initial increase is so short as to 

 escape detection ; which may be true, but the burden 

 of proof is on M. Bohn. 



