ANIMAL BEHAVIOUR IN TERMS OF PERCEPTION         97
The most systematic attempt to describe organic behaviour in
general, possibly including human behaviour, on the tropism
theory is that of Loeb, and therefore I will take his exposition of
the theory as an example of an extreme application of it.
If a plant seedling is placed in a horizontal position under
conditions in which continued growth is possible, the young root
curves downwards and the shoot curves upwards. This is an
example of a tropism. The root is positively, the shoot negatively,
geotropic. Many insects and other animals, if illuminated by a
beam of light, orientate themselves so that they move either
towards the source of light or away from it. This is also a
tropism, in this case phototropism, which, as in all tropisms, can
be positive or negative. As, however, there are great physiological
differences between the response involved in a plant tropism, in
which the directed movement is due to differential growth, and
the response involved in an animal tropism which is brought
about by its motor organs, the latter are often distinguished as
taxis; for example, an animal may be positively or negatively
phototactic. Following Loeb, however, I will speak of all such
responses, whether of plants or animals, as tropisms.
A tropism, then, is a response to some general feature of the
environment, such as gravity, light, heat, contact with solids, and
so forth. Moreover, a tropistic response is conceived by Loeb as
an automatic one, the causes of which are completely analysable
into physico-chemical processes. Loeb builds up his general
'tropism theory of animal conduct' from its simplest manifestation,
the response to light by a bilaterally symmetrical animal. He
conceives that the orientation and movement to the light of a
positively phototropic insect illuminated from one side is brought
about in the following way: The greater velocity of photochemical
reactions in the eye on the illuminated side causes a greater
intensity of nerve impulses to travel down the nerves to the muscles
of the opposite side, which therefore assume a greater tension, and
contract more forcibly, than the muscles on the illuminated side.
Thus when the animal begins to move, it will move in a curve
towards the source of light owing to the stronger action of the
legs on the shaded side. It will continue to move in a curve till
its body is parallel with the beam of light. In this position the two
eyes are equally illuminated and therefore the legs on the two
sides are equally stimulated and act with equal force. Conse-
quently the animal now moves in a straight line towards the source
of light