266 Influence of environment on animals 



The faint light from the sky is sufficient to cause animals which 

 are in a high degree positively heliotropic to move vertically upwards 

 towards 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 heliotropism, and 

 slowly sink down or become negatively heliotropic and migrate 

 actively downwards. 



These experiments have also a bearing upon the problem of the 

 inheritance of instincts. The character which is transmitted in this 

 case is not the tendency to migrate periodically upwards and down- 

 wards, 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 sense and intensity of the heliotropism 

 of these animals. It is of course immaterial for the result, whether 

 the carbon-dioxide or any other acid diffuse into the animal from the 

 outside or whether they are produced inside in the tissue cells of the 

 animals. Davenport and Cannon found that Daphniae, which at the 

 beginning of the experiment, react sluggishly to light react much 

 more quickly after they have been made to go to the light a few 

 times. The writer is inclined to attribute this result to the effect of 

 acids, e.g. carbon-dioxide, produced in the animals themselves in 

 consequence of their motion. A similar effect of the acids was shown 

 by A. D. Waller in the case of the response of nerve to stimuli. 



The writer observed many years ago that winged male and female 

 ants are positively heliotropic and that their heliotropic sensitiveness 

 increases and reaches its maximum towards the period of nuptial 

 flight. Since the workers show no heliotropism it looks as if an 

 internal secretion from the sexual glands were the cause of their 

 heliotropic sensitiveness. V. Kellogg has observed that bees also 

 become intensely positively heliotropic at the period of their wedding 

 flight, in fact so much so that by letting light fall into the observation 

 hive from above, the bees are prevented from leaving the hive through 

 the exit at the lower end. 



We notice also the reverse phenomenon, namely, that chemical 

 changes produced in the animal destroy its heliotropism. The cater- 

 pillars of Porthesia chrysorrhoea are very strongly positively helio- 

 tropic when they are first aroused from their winter sleep. This 

 heliotropic sensitiveness lasts only as long as they are not fed. If 

 they are kept permanently without food they remain permanently 

 positively heliotropic until they die from starvation. It is to be 

 inferred that as soon as these animals take up food, a substance or 

 substances are formed in their bodies which diminish or annihilate 

 their heliotropic sensitiveness. 



