e reactions of which are
not complicated by associative memory, or, as it may preferably be
termed, associative hysteresis.

The simplest type of instincts is represented by the purposeful motions
of animals to or from a source of energy, e.g. light; and it is with
some of these that we intend to deal here. When we expose winged aphides
(after they have flown away from the plant), or young caterpillars of
Porthesia chrysorrhoea (when they are aroused from their winter sleep)
or marine or freshwater copepods and many other animals, to diffused
daylight falling in from a window, we notice a tendency among these
animals to move towards the source of light. If the animals are
naturally sensitive, or if they are rendered sensitive through the
agencies which we shall mention later, and if the light is strong
enough, they move towards the source of light in as straight a line as
the imperfections and peculiarities of their locomotor apparatus will
permit. It is also obvious that we are here dealing with a forced
reaction in which the animals have no more choice in the direction
of their motion than have the iron filings in their arrangement in a
magnetic field. This can be proved very nicely in the case of starving
caterpillars of Porthesia. The writer put such caterpillars into a glass
tube the axis of which was at right angles to the plane of the window:
the caterpillars went to the window side of the tube and remained there,
even if leaves of their food-plant were put into the tube directly
behind them. Under such conditions the animals actually died from
starvation, the light preventing them from turning to the food, which
they eagerly ate when the light allowed them to do so. One cannot say
that these animals, which we call positively helioptropic, are attracted
by the light, since it can be shown that they go towards the source
of the light even if in so doing they move from places of a higher to
places of a lower degree of illumination.

The writer has advanced the following theory of these instinctive
reactions. Animals of the type of those mentioned are automatically
orientated by the light in such a way that symmetrical elements of their
retina (or skin) are struck by the rays of light at the same angle.
In this case the intensity of light is the same for both retinae or
symmetrical parts of the skin.

This automatic orientation is determined by two factors, first a
peculiar photo-sensitiveness of the retina (or s