1919] Esterhj: Reactions of Various Plankton Animals 13 



however (for example, 1902, p. 627), that alterations of viscosity are 

 not the only causes of vertical movements. Tropisms play a part but 

 a subordinate one. 



This is hardly the place to go more deeply into Ostwald 's explana- 

 tion, especially since my own experiments were along very different 

 lines than those suggested by his work. It does not seem to me, how- 

 ever, that plankton organisms can be regarded as passive bodies that 

 are carried up and down through physical agencies. Those forms that 

 I have watched swim up when they ascend. Although they may drop 

 down passively, this does not occur as a result of change of tempera- 

 ture so far as I can see; temperature has an effect but not in the way 

 set forth by Ostwald. 



In concluding the review of the literature, it seems necessary to 

 note the somewhat unusual views expressed by Franz in a number of 

 papers (1911, 1912, 1913). He holds that vei'tical migrations do not 

 occur; that the reason for the larger number at the surface at night, 

 as shown by collections, is that the animals can not see the nets then 

 so as to avoid them. According to his view, the animals are really as 

 abundant at the surface during the day as during the night but they 

 escape the nets when it is light enough for them to see (1911, p. 10). 

 I believe that the views Franz expresses are untenable so far as they 

 relate to carefully conducted field investigations, but he has done well 

 to point out that the results of laboratory experiment alone do not 

 show what goes on in nature. For example (1913, p. 265) he states 

 that in his opinion the observations concerning the phototaxis of 

 Bala n us larvae and similar forms do not offer any ground for assum- 

 ing that plankton animals perform periodical vertical migrations. 

 Phototactic phenomena in the laboratory give only a broken picture 

 of behavior in the open. When he proceeds, however, to argue that 

 because experimental results do not show what takes place in nature, 

 the diurnal movement does not take place, he goes too far. He gives 

 a good many reasons why laboratory phototaxis can not be identified 

 with migration movements in the open and says (1912, p. 499) : "Diese 

 wohl einschneidende Kritik der Lehre der Phototaxis bei Tieren ent- 

 zieht zugleich der Annahme von regelmassigen vertikalen Wander- 

 ungen der Planktontiere eine ihrer wiehtigsten Stutzen. ' ' The photo- 

 taxis (in the laboratory) is at most a "flight movement" (fluchtbe- 

 wegung) which can only be of short duration in the open. It will be 

 noted that Franz passes from the statement that a supposed habit 

 can not be explained in a particular way to the argument that, for 

 such a reason, the existence of the habit is questionable. 



