HELIOTROPISM OF ANIMALS 35 
mals, when a few have collected in a spot the others on 
arriving hold fast to the sides of those already there. An 
animal at:rest acts upon a creeping one as a convex edge. 
On the other hand, I have never observed that the animals 
within the cubical box collect on concave edges. From this 
it follows that the friction of gliding over the convex corners 
is the source of the stimulation which compels the animal to 
come to rest there; in moving over the concave corners this 
friction, of course, does not take place. 
These three forms of irritability control mainly the daily 
life of the animals. We find them in great numbers in 
fruit trees and bushes, where they pass the winter in their 
nests; as soon as the warm weather comes, they leave their 
nests. Positive heliotropism and negative geotropism com- 
pel them to creep upward to the tips of branches, and contact- 
irritability holds them fast on the small buds. We can 
easily show that neither smell nor a special mystical 
“instinct” leads the animals to the buds, as we are able to 
compel them by the aid of light to starve in close proximity 
to food. The animals move to the window side or to the 
top of a test-tube in which they are kept. If then a branch 
covered with buds is pushed into the test-tube on the room 
side, the animals nevertheless remain where light and gravi- 
tation have compelled them to go and are holding them. If, 
however, they once are on the buds, the latter act as a 
stimulus which may be even stronger than the light. It is 
in such a case impossible to draw the animals away from the 
food by means of light. 
All these forms of irritability can best be demonstrated 
on animals which have just left the nest in which they have 
spent the winter, and which have not yet eaten anything. 
As soon as they have eaten and are about to moult, their 
irritability decreases, and at the time of moulting it is almost 
impossible to show any effect of light or gravity upon them. 
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