Influence of Environment on Animals 221 



the water also. If the water contammg the Daphniae is cooled 

 and at the same time carbon dioxide added, the animals which 

 were before indifferent to light now become most strikingly 

 positively heliotropic. Marine copepods can be made posi- 

 tively heliotropic by the lowering of the temperature alone, or 

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



These data have a bearing upon the depth migrations of 

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

 and the writer. It is well known 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 motions 

 are determined to a large extent, if not exclusively, by the 

 heliotropism of these animals. Since the consumption of carbon 

 dioxide by the green plants ceases toward evening, the tension 

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

 of inducing positive heliotropism or increasing its intensity. 

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

 is lowered and this also increases the positive heliotropism in the 

 organisms. 



The faint hght from the sky is sufficient to cause animals 

 which are in a high degree positively heUotropic to move 

 vertically upward toward the light, as experiments with such 

 pelagic animals, e.g., copepods, have shown. 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 heUotropism, and slowly sink down or become 

 negatively heliotropic and migrate actively downward. 



These experiments have also a bearing upon the problem 

 of the inheritance of instincts. The character which is trans- 

 mitted in this case is not the tendency to migrate periodically 

 upward and downward, but the positive heliotropism. The 

 tendency to migrate is the outcome of the fact that periodically 

 varying external conditions induce a periodic change in the 



