1919] Esterly: Reactions of Various Planktmi A initials 3 



the animals pass into warmer and warmer water. From the surface 

 to a depth of about thirty meters there is a slight decrease in salinity, 

 but from thirty meters down the salinity gradually increases. The 

 light intensity during the day grows less as the distance beneath the 

 surface increases; this is probably true for the night also; but for that 

 time the difference in degree of illumination at the surface and at 

 lower levels is so much less than during the day that it is practically 

 negligible, unless phosphorescence makes light that is more intense 

 below the surface. Since plankton animals are commonly found in 

 deeper water by day and nearer the surface by night, they pass through 

 a range of temperatures and densities as they move up and down. 

 Furthermore, during the day they are in water of higher salt con- 

 centration and lower temperature and in light of relatively low inten- 

 sity, compared with the surface. "While the illumination at the sur- 

 face is low during the night also the temperature is higher and the 

 salinity lower than the animals encounter by day, although forms 

 that do not descend during the day farther than about thirty meters 

 will be in water of less salinity than at the surface. 



Now if natural surroundings are to be simulated in experiments 

 conducted for the purpose already mentioned, it is a comparatively 

 simple matter soio arrange conditions that if the animals move down- 

 ward they meet with lower temperature, higher salinity, and decreased 

 light intensity, or with the reverse of these three conditions if they 

 ascend. It is needless to point out that there are some oceanic condi- 

 tions that can not be reproduced in the laboratory, such as great depth 

 of water, tidal currents, or distance from shore. But it is easy to pro- 

 vide a temperature gradient with cold water at the bottom of a column. 

 If a light is placed at the top of the column the intensity will be 

 greatest at the surface and progressively less below the surface. Or 

 it may be arranged that cold water of higher salinity shall be found 

 at the lower part of a cylinder where the light intensity is measurably 

 less than toward the top. On the other hand, experimental conditions 

 can be secured in the laboratory which would not be met by the 

 organisms in the sea. An example of this is lighting a vertical con- 

 tainer from below. It is permissible, of course, to introduce unnatural 

 conditions but only for the purposes of interpreting activities ob- 

 served under the more natural conditions. On the whole, however, 

 the experimental conditions were patterned after the general oceanic 

 environment as regards light, temperature, and salinity. 



This experimental study of diurnal migration was undertaken only 

 after a number of years of field investigation by the Scripps Institu- 



