220 The Mechanistic Conception of Life 



at the same angle, there is no more reason for the animal to 

 deviate from this direction and it will move in a straight line. 

 All this holds good on the supposition that the animals are 

 exposed to only one source of light and are very sensitive to 

 light. 



Additional proof for the correctness of this theory was 

 furnished through the experiments of G. H. Parker and S. J. 

 Holmes. The former worked on a butterfly, Vanessa antiope, 

 the latter on other arthropods. All the animals were in a 

 marked degree positively heliotropic. These authors found 

 that if one cornea is blackened in such an animal, it moves 

 continually in a circle when it is exposed to a source of light, 

 and in these motions the eye which is not covered with paint is 

 directed toward the center of the circle. The animal behaves, 

 therefore, as if the darkened eye were in the shade. 



h) The production of positive heliotropism hy acids and other 

 means and the periodic depth migrations of pelagic animals. — 

 When we observe a dense mass of copepods collected from a 

 fresh-water pond, we notice that some have a tendency to go to 

 the light while others go in the opposite direction and many, 

 if not the majority, are indifferent to light. It is an easy matter 

 to make the negatively heliotropic or the indifferent copepods 

 almost instantly positively heliotropic by adding a small but 

 definite amount of carbon dioxide in the form of carbonated 

 water to the w^ater in which the animals are contained. If the 

 animals are contained in 50 c.c. of water it suffices to add from 

 3 to 6 c.c. of carbonated water to make all the copepods energeti- 

 cally positively heliotropic. This heliotropism lasts about 

 half an hour (probably until all the carbon dioxide has again 

 diffused into the air). Similar results may be obtained with 

 any other acid. 



The same experiments may be made with another fresh- 

 water crustacean, namely Daphnia, with this difference, how- 

 ever, that it is as a rule necessary to lower the temperature of 



