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SCIENCE 



[N. S. Vol. LII. No. 1344 



lower levels; by day the abundance is less at 

 liig'ber levels and greater at lower levels. One 

 form may have a vertical range from the 100 

 fathom to the 200 fathom mark and another 

 may range frona the surface to 50 fathoms. 

 The increase in numbers in the upper strata 

 at night is believed to be due to upward 

 movement of the animals from lower levels; 

 the increase in numbers below the surface dur- 

 ing the day is held to be due to descent from 

 higher levels. The whole phenomenon is 

 spoken of as diurnal migration. 



The explanation of such movements has 

 often been attempted. The one that has been 

 most in vogue is based on change in direction 

 of movement as some external condition is 

 altered. That is, reactions are involved in the 

 varying relations between the environment and 

 the distribution of the organisms. Since the 

 vertical movement is rhythmic and corresponds 

 to the change between day and night it seems 

 reasonable to suppose that the direction of 

 movement changes as the light intensity 

 changes; the animals might move toward a 

 light of low intensity and away from one of 

 higher intensity. Again, evidence has been 

 presented showing that the geotropism of some 

 forms changes with the light intensity. It has 

 been thought, furthermore, that alterations in 

 temperature or salinity are accompanied by 

 reversals of direction of movement, either with 

 relation to a source of light or to gravity. 

 There is reason for this view since the organ- 

 isms actually pass through different tempera- 

 tures and salinities as they move up and down. 

 I The foregoing possibilities all exist. But 

 such questions as these have arisen : How gen- 

 erally do the possibilities apply ? Are they ap- 

 plicable to all plankton animals ? Do reversals 

 in direction of movement really take place as 

 it is possible they may? An investigation of 

 such questions occupied the time of the writer 

 for a year at the Scripps Institution. The 

 reactions of diilerent plankton animals were 

 studied, with the special point in mind of ap- 

 plying the results to the diurnal migration 

 problem. The attempt was made to find out 

 for each species whether movement is toward 

 or away from a weaker or stronger light; 



whether specimens ascend or descend in dark- 

 ness as compared with diffuse light; whether 

 locomotion in a vertical direction is toward or 

 away from a light at the top of a column of 

 water; whether direction of movement under 

 any of the foregoing conditions changes if the 

 temperature or salinity is changed. 



It would be out of place to go into details 

 here, but it should be said that experimental 

 data that may show why the diurnal movement 

 takes place were not obtained except in the ease 

 of the chsetognath, Sagitta hipunctata; this 

 form is one of spven that were used consist- 

 ently. There is good evidence from field 

 records that all of the species used in the ex- 

 periments perform the diurnal migration. 

 Furthermore, the field data from the San Diego 

 region relating to one of the copepods and to 

 Sagitta have been more completely worked out 

 than for any other marine plankton animals 

 anywhere. The relation between abundance or 

 scarcity of these forms and external condi- 

 tions revealed by collecting records, is thor- 

 oughly well known. 



' While the experiments that have been men- 

 tioned did not show how behavior in the labor- 

 atory is related to diurnal migration, they did 

 bring to light matters that are certain to af- 

 fect the interpretation of experimental results 

 as applied to an explanation of a natural 

 habit. It may be suggested, also, that these 

 matters will probably enter into any study of 

 reactions whose purpose is to discover the rea- 

 sons for a given sort of behavior under natural 

 conditions. The following are the points which 

 seem to be of general importance. 



1. Specificity in behavior. There is no doubt 

 that a general explanation of diurnal migra- 

 tion is possible only when the different species 

 have been studied in the laboratory; and it 

 may not be possible even then to account for 

 the diurnal migration as a general occurrence 

 in the plankton. Various animals perform the 

 migration together, but the reasons are differ- 

 ent for the different forms. " Blanket expla- 

 nations " have been given which are mislead- 

 ing because the reaction differences between 

 species were not appreciated. One should not 

 attempt to account for the vertical migration. 



