1919] Esterly: Reactions of Various Plankton Animals 75 



movement in directive light as the temperature is decreased. It is 

 important to note in this connection that the upward migration in 

 nature does not take place, in general, in the full intensity of daylight ; 

 at any rate the accumulation at the surface occurs after dark in most 

 cases. If, therefore, it is found in experiments that there is relatively 

 more upward movement toward a light when the temperature of the 

 water is lower than when it is higher, we ought not to apply this result 

 to natural conditions unless we know that the results of field work 

 are similar to those of the laboratory. Since we find that the upward 

 movement in nature occurs in light of low intensity or in what is prac- 

 tically darkness, we should look to the results of experiments on geo- 

 tropism in dim light or in darkness if we are to arrive at an explana- 

 tion of the diurnal habit from laboratory tests. 



To What Extent Will Behavior Under Recurrent External 

 Conditions Explain the Diurnal Migration? 



The general facts about the diurnal distribution of plankton or- 

 ganisms are these : at night there is greater abundance of a given 

 species at higher levels, less abundance at lower levels ; by day the 

 abundance is less at higher levels, and greater at lower levels. One 

 form may have a vertical range from the 100 fathom to the 200 fathom 

 level and another may range between the surface and 100 fathoms. In 

 each ease, however, the general statement made above will apply, 

 though it makes the matter of diurnal migration seem much simpler 

 than it really is. 



What responses are involved in such varying relations between 

 the distribution of organisms and the environment 1 Since the vertical 

 movement is rhythmic and corresponds to the change between day and 

 night it is natural to expect that in the laboratory diurnal migration 

 will be simulated as the light intensity changes. The upward and 

 downward phases should take place under artificial conditions that are 

 similar in general, if not in detail, to the natural conditions under 

 which the animals ascend and descend if the laboratory results are to 

 be applicable to the behavior in nature. For instance, the ascent in a 

 vertical tube should take place in darkness if it is known that the 

 species under experiment is most abundant at the surface of the sea 

 during the night. Or, if the animals in an experiment remain at the 

 top of a column of water at 10° C but not at 17° C it would be 



