REACTIONS TO HEAT AND COLD. 25 



rapid movements many of them by chance enter the cooler region. 

 They do not react at all as they enter, but continue across. On 

 coming to the other side of the drop, however, they do react, by back- 

 ing and turning toward one side (the aboral). They react whenever 

 they come to the boundary of the cooled region ; hence they do not 

 leave it. In every respect their behavior is like that seen when Para- 

 mecia collect in a drop of weak acid, and I believe there is no longer 

 anyone who holds to the orientation theory for the gathering of Para - 

 mecium in chemicals. 



As in the case of chemicals, it may be demonstrated to the eye in the 

 following manner that the method above described suffices to account 

 for the gatherings. On the upper surface of the cover glass is marked 

 a small ring in ink. By confining the attention to this ring it is easily 

 seen that in the heated preparation of Paramecia many individuals 

 cross the ring every instant, so that, if these could all be stopped in 

 the ring, a dense aggregation would soon result. Then the region 

 within the ring is cooled by placing a drop of ice water on the cover 

 above it. The Paramecia continue to swim just as before, save that 

 they no longer pass out of the ring after swimming in, as they did at 

 first. In this way a dense collection is soon formed. 



Mendelssohn (1902, b, p. 487) finds it inexplicable why the Para- 

 mecia should form dense aggregations at the optimum temperature. 

 He says that they execute " only some insignificant movements" in 

 this region, not swimming away. On the theory of thermotaxis held 

 by Mendelssohn this is perhaps inexplicable, but this, it seems to me, 

 is only because the theory is incorrect. Such collections are due to 

 precisely the same factors as the rest of the reaction to heat and cold 

 and are clearly intelligible when the nature of the reaction, as described 

 above, is taken into consideration.* 



In a former paper (Jennings, 1899, p. 336), after giving a brief 

 account of the reaction method above described, I pointed out that this 

 method does not demand a sensitiveness to such minute differences in 

 temperature as does Mendelssohn's theory, and that therefore the sensi- 

 tiveness to temperature differences may have been overestimated. 



* Mendelssohn (1902, 3, p. 487) supposes that I would explain these gatherings 

 at the optimum temperature through the collection of Paramecia in CO 2 pro- 

 duced by themselves, and shows that this would not account for the phenomena 

 observed in these cases, though he confirms the fact of the collections in CO 2 . 

 But I have by no means maintained that such collections can be produced only 

 by CO 8 ; on the contrary, I have given an account of many different agencies 

 that will give rise to such collections, and have especially described the fact that 

 collections are formed in a warmed region through exactly the same reaction by 

 which they are formed in CO 2 . (Jennings, 1899, P- 3 I 5) 



