Influence of Environment on Animals 221 



the water also. If the water containing the Dujjh/iiac is cuuled 

 and at the same time carbon (Hoxide added, the aiiininls wliieli 

 were before indifferent to Hght now ix'coinc most strikinj^ly 

 positively heliotropic. Marhie eo])ei)ods ean be made posi- 

 tively heliotropic by the lowering of the tem])erature alone, or 

 by a sudden increase in the concentration of the sea-water. 



These data have a bearing upon the dej^th migration.s of 

 pelagic animals, as was pointed out years ago by Theo. T. Clroom 

 and the writer. It is well kno^^^l that many animals living 

 near the surface of the ocean or fresh-water lakes, have a 

 tendency to migrate upward toward evening and downward 

 in the morning and during the day. These periodic motioiLs 

 are determined to a large extent, if not exclusively. })y the 

 heliotropism of these animals. Since the consumi)tion of carbon 

 dioxide by the green plants ceases toward evening, the tension 

 of this gas in the water must rise and this nmst have the effect 

 of inducing positive heliotropism or increasing its mtensity. 

 At the same time the temperature of the water near the surface 

 is lowered and this also increases the positive heliotroiiism in the 

 organisms. 



The faint light from the sky is sufficient to cause animaLs 

 which are in a high degree positively heliotropic to move 

 vertically upward toward the light, as experiments with such 

 pelagic animals, e.g., copepods, have sho\Mi. When, in the 

 morning, the absorption of carbon dioxide by the green algae 

 begins again and the temperature of the water rises, the animals 

 lose their positive heliotropism, and slowly sink down or become 

 negatively heliotropic and migrate actively downward. 



These experiments have also a bearing ui^mi the problem 

 of the inheritance of instincts. The character which is tr:uis- 

 mitted in this case is not the tendency to migrate iK^riodically 

 upward and do^vnward, but the positive heliotropism. Tin- 

 tendency to migrate is the outcome of the fact that lu-riodically 

 varying external conditions induce a jieriodic change in the 



