1913] Gee: Behavior of Leeches 265 



In the accompanying table (see table VI), even where the 

 zeros are recorded in the two-minute interval responses, there 

 is to be observed in the animal a slowing-up of movement with- 

 out actual cessation. Mention of the fact is made to show that 

 even in these cases we have a partial recuperation of the original 

 condition of the animal evidenced at the beginning of the series 

 of applications of the stimulus. The acclimatization to shadows 

 is to be observed as much more readily effected than in the case 

 of slight jars. 



3. General Discussion of Acclimatization Results 



The phenomenon of acclimatization is such a familiar one, 

 and the numerous cases of it have been cited so frequently in 

 the literature on animal behavior, that only a reference to many 

 of them is necessary to recall the details to the mind of the 

 reader. In Stentor Jennings (1902) found that the tirst few 

 light contact stimuli caused a contraction of the animal, but 

 subsequent ones of the same general intensity failed to produce 

 this effect. Wagner (1904) has shown a similar condition to 

 hold in Hijdra. Holmes (1911) found "that light mechanical 

 stimulation of the anterior end of Loxophyllum causes at first 

 a ready response ; several repetitions in close succession diminish 

 the responsiveness so that a much stronger stimulus often fails 

 to produce any noticeable effect. In a few seconds, however, 

 recovery is apparently complete, and the animal is as responsive 

 as before." Somewhat similar phenomena have been found in 

 planarians (Pearl, 1903), and the experiments of Mrs. Yerkes 

 (1906) and Ilargitt (1907) on tubicolous annelids have been 

 mentioned in a preceding paragraph. 



Dr. Holmes (1911) in discussing the causative factors in 

 this tj'pe of modifiability summarizes the three possible expla- 

 nations as follows: "Gradual cessation of response to a given 

 stimulus may be due (1) to a fatigue of the motor apparatus, 

 (2) to a dulling of the sensibility of the receptors, or (3) perhaps 

 to other states of the organism. A considerable part of the cases 

 of cessation of reaction to a given stimulus are possibly due, as 

 Bohn contends, to a 'fatigue sensorielle' or dulling of sensibility. 



