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, eg. 
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. 
