40 University of California Publications in Zoology [Vol.19 



of these various results, it appears that the upward migration requires 

 water of relatively low temperature and light at the surface. 



But, according to the published results of the field work (Esterly, 

 1912, pp. 282-285), the accumulation at the surface is greatest from 

 8 p.m. to 12 p.m. It is reasonable to suppose that some of the upward 

 journey is accomplished in very dim light or in darkness, and it is 

 difficult to account for such behavior on an experimental basis. If we 

 grant that the organisms in descending reach water of a temperature 

 so low that they begin to ascend, I do not see how the continuation of 

 the ascent is to be accounted for after the animals reach the warmer 

 water at higher levels. The initial stages of the upward migration 

 may take place in the colder water of the deeper strata, but some other 

 factor is apparently necessary in order to account for the completion 

 of the journey to the surface. This factor, as shown by the experi- 

 ments, may be the physiological rhythm. It is possible that the ascent 

 may be accomplished in the absence of directive stimuli. 



If the descent takes place as day begins to dawn it is readily 

 accounted for, on the basis of these experiments, by negative photo- 

 tropism at the temperatures that prevail at the surface and by posi- 

 tive geotropism in light from above. Our field data show, however, 

 that these copepods forsake the surface several hours before dawn 

 (Esterly, 1912, p. 284), though our observations are admittedly in- 

 conclusive. If more complete information should establish this point 

 the positive geotropism in darkness might explain the descent and 

 there would be no need to attribute anything to a response to light. 

 When the "physiological state" that prevails during the time that 

 the animals show negative geotropism wears off or is changed the 

 descent will begin. 



It is certain that under natural conditions large numbers of these 

 animals arrive at the surface in the dark and continue to be obtained 

 there during several hours of darkness. Their ascent begins in water 

 of lower temperature than that at the surface, and the water becomes 

 continually warmer at higher levels until temperatures are encountered 

 in which there should be negative phototropism, as shown by the 

 laboratory results. The problem is to explain the ascending into 

 warm water in darkness. I believe that the facts brought out in con- 

 nection with the physiological or metabolic rhythm offer the best basis 

 for the desired explanation. The cause of the rhythm is, in turn, a 

 separate question. 



An explanation of the diurnal migration of Galanus based on geo- 



