March 31, 1911] 



SCIENCE 



Alb 



times along different lines of descent. In- 

 telligence in tlie vertebrates doubtless arose 

 independently from that of the insects, and 

 the intelligence exhibited here and there 

 among the mollusks probably arose inde- 

 pendently along a third line of develop- 

 ment. Intelligence makes its appearance 

 at a certain stage of organization along 

 whatever line such a stage may have been 

 reached. 



Up to the point at which the power of 

 associative memory becomes manifest there 

 has been progress along many lines which 

 has prepared the way for the evolution of 

 this new faculty. Behavior has not only 

 become more complex, but it has become 

 more plastic and capable of easy modifica- 

 tion to suit new conditions. The lower 

 organisms do not always react in a par- 

 ticular way to a given stimulus. What 

 reaction occurs may depend upon the num- 

 ber of previous stimulations, the supply of 

 food, exposure to different environing con- 

 ditions, and numerous other factors which 

 influence the internal state of the organ- 

 ism. The behavior of many lower animals 

 is plastic and adaptive to a remarkable de- 

 gree, and to a superficial consideration 

 often gives the appearance of a consider- 

 able degree of intelligence, without there 

 being any detectable power of associative 

 memory. This plastic and varied behavior 

 not only simulates intelligence, but it se- 

 cures for the organisms many of the ad- 

 vantages which intelligence confers. It 

 adapts the animal to a more varied envi- 

 ronment, and gives it the power of meeting 

 a given situation in more than one way, so 

 if one kind of response does not suit, an- 

 other may be more successful. Let us 

 glance briefly at some of the ways in which 

 behavior may be modified. 



A very general change of behavior in its 

 organisms consists in the habituation to 

 any stimulus which is repeated at suffi- 



ciently close intervals so that the organism 

 no longer responds to it. This is shown 

 even among the protozoa. A Stentor or a 

 Loxophyllum subjected to a light mechan- 

 ical 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 ex- 

 tend over two or three seconds. Similar 

 effects of repeated stimulation but of 

 longer duration have been observed in 

 Hydra, several species of sea-anemones, 

 planarians, annelids and various other 

 lower invertebrates. 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 oc- 

 curs when the response to a given stimulus 

 is increased instead of diminished with 

 repeated applications — a result which sug- 

 gests the effect of the summation of stim- 

 uli. At times, as Bohn found in Cerian- 

 thus, there is an initial increase of re- 

 sponsiveness followed by a dulling of sensi- 

 tivity. Bohn has attempted to subsume 

 the effects of repeated stimulation under a 

 general "law" to the effect that stimula- 

 tion always produces at first increase of 

 sensitivity to be followed later by a de- 

 crease. Sometimes, as Bohn claims, the 

 initial increase is so short as to escape de- 

 tection ; which may be true, but the burden 

 of proof is on M. Bohn. 



Repetition of a stimulus may call forth 

 not only quantitative differences of re- 

 sponse, but it may evoke responses of very 

 different character. Animals are fre- 

 quently provided with several modes of 

 reacting to a given stimulus which may be 

 called into play one after the other. Jen- 

 nings has shown that if a Stentor is sub- 

 jected to a light mechanical stimulus by 

 causing fine particles of India ink to fall 

 upon its disk from a capillary pipette it 



