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 helioptropic 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 caterpillars of
Porthesia chrysorrhoea are very strongly positively heliotropic 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.

The heliotropism of animals is identical with the heliotropism of
plants. The writer has shown that the experiments on the effect of acids
on the heliotropism of copepods can be repeated with the same result in
Volvox. It is therefore erroneous to try to explain these heliotropic
reactions of animals on the basis of peculiarities (e.g. vision) which
are not found in plants.

We may briefly discuss the question of the transmission through the sex
cells of such instincts as are based upon heliotropism. This problem
reduces itself simply to that of the method whereby the gametes transmit
heliotropism to the larvae or to the adult. The writer has expressed the
idea that