GENERAL BIOLOGY 



703 



fish or scopelidse, while at 60 metres there were several 

 copepoda, and no scopelidae. In the same place, during the 

 night, we obtained at the surface a rich collection of copepoda, 

 numerous scopelidae, and thirteen black fishes {Astronestkes 

 niger). These instances furnish conclusive proof of vertical 

 migrations of considerable extent. 



Ostwald, after studying the variations in the viscosity of the 

 water from time to time, has made an attempt to explain the 

 vertical migrations as due entirely to physical laws. During the 

 twenty-four hours certain changes occur in the temperature of 

 the ocean surface, and the viscosity of the water is, as we have 

 seen, largely dependent on temperature. According to Buchan, 

 the mean diurnal fluctuation of the surface temperature, as 

 shown by the "Challenger" observations, was in mid North 

 Atlantic 0.8' Fahr., in mid South Atlantic also 0.8' F., in mid 

 North Pacific 1.0° F., and in mid South Pacific 0.9° F, ; near 

 the equator both in the Atlantic and Pacific the diurnal range is 

 only 0.7° F. The mean daily range deduced from the whole of 

 the "Challenger" observations during the three years and a 

 half is 0.8° F.^ 



According to Krlimmel ^ the daily range of temperature 

 occurring in the surface waters of the open ocean amounts to 

 about 0.5° C. ; in the North Atlantic 0.59° C. Although several 

 investigators, like Aime and Hensen, tackled the problem we 

 have very little knowledge regarding the daily changes at 

 different depths. From Krlimmel I give the following differ- 

 ences found by Aime between evening and morning at different 

 depths in the Mediterranean : — 



1 Phys. Chem. Chall. Exp., Part v. p. 6, 1889. 

 Otto Kriimmel, Handbuch der Ozeanographie , Bd. I, Leipzig, 1907. 



