ir.) Similar results may be
obtained with any other acid.

The same experiments may be made with another freshwater crustacean,
namely Daphnia, with this difference, however, that it is as a rule
necessary to lower the temperature of the water also. If the water
containing 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
positively 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
freshwater lakes, have a tendency to migrate upwards towards evening and
downwards 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 towards 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 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 downwards,
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