264 Influence of environment on animals 
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 
heliotropic, are attracted by the light, since it can be shown that 
they go towards the source of 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 skin), and second a 
peculiar nervous connection between the retina and the muscular 
apparatus. In symmetrically built heliotropic animals in which the 
symmetrical muscles participate equally in locomotion, the symmetrical 
muscles work with equal energy as long as the photo-chemical pro- 
cesses in both eyes are identical. If, however, one eye is struck by 
stronger light than the other, the symmetrical muscles will work 
unequally and in positively heliotropic animals those muscles will 
work with greater energy which bring the plane of symmetry back 
into the direction of the rays of light and the head towards the 
source of light. As soon as both eyes are struck by the rays of light 
at the same angle, there is no more reason for the animal to deviate 
from this direction and it will move in a straight line. All this holds 
good on the supposition that the animals are exposed to only one 
source of light and are very sensitive to light. 
Additional proof for the correctness of this theory was furnished 
through the experiments of G. H. Parker and S. J. Holmes. The 
former worked on a butterfly, Vanessa antiope, the latter on other 
