REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 69 



less illuminated end. But this involves the capability of making fine 

 distinctions, and a considerable degree of intelligence in deciding what to 

 do under the peculiar circumstances. The experiment with the swarm- 

 spores shows that they are incapable of such fine discrimination, or that 

 they are not sufficiently intelligent to know what to do under the circum- 

 stances. They give no indication that they notice the greater illumina- 

 tion after having passed to the end away from the light. Their action 

 may be considered perhaps in a certain sense as a u mistake," but it 

 is a mistake which even the highest organism would make, and which 

 could be corrected only after experience of its results. 



The results of our study of the light reaction in ciliates and flagel- 

 lates lead to conclusions which stand in sharp contrast with certain 

 general conclusions in Radl's recent extensive and interesting paper on 

 Phototropism (Radl, 1903). Radl reaches the somewhat extraordinary 

 conclusion that light orients organisms by exercising an actual 

 mechanical pressure upon them. This pressure necessarily disturbs 

 the equilibrium of the body, which is then compelled to change posi- 

 tion until equilibrium is restored ; the organism is then oriented. The 

 orientation is a consequence of the interplay of two sets of forces, inner 

 and outer ; these cannot be in equilibrium until the body has taken a 

 certain position with reference to the pressure exercised by the light 

 (/. c., pp. 151 ff.) The actual turning which induces orientation must 

 be due to the action of a pair of forces (/. c., p. 148). One of these 

 forces is the pressure produced by the light 



Orientation produced in the manner described in the present paper 

 for the reaction of ciliates and flagellates to light, and in the preceding 

 paper for the reaction to heat, could of course not be brought about in 

 the manner supposed by Radl. One of Radl's chief arguments for his 

 view is that " no observation thus far shows that the final orientation 

 is attained by a trial or after an oscillation, but it takes place auto- 

 matically"* (/. c., p. 141). 



The observations on ciliates and flagellates given in the present paper 

 show conclusively that the orientation in these cases is brought about 

 through repeated trials. In the statement quoted above Radl has over- 

 looked certain other cases. Thus Strasburger, as we have seen (p. 59), 

 states that after the direction of the light is changed Haematococcus 

 becomes reoriented " nach verschiedenen Schwankungen" (Strasbur- 

 ger, 1878, p. 24). Radl himself refers on a previous page (p. 100) to 

 Strasburger's observation of the oscillating movement of swarm-spores 

 under the influence of a variation in light intensity; Rothert (1901, 



*"Keine bisherige Beobachtung zeigt ferner, dass die schliessliche Orientie- 

 rung etwa durch eine Priifung oder nach einem Schwanken erzielt wiirde, 

 sondern sie folgt automatisch." 



