96 SECTIONAL ADDRESSES 
their wings. Some animals find safety in immobility, or in the protection 
afforded by a hard shell or carapace, or an armour of spines ; the tube- 
worm retracts its tentacles like a flash and may close the tube up with 
a stopper. 
In many of these cases the animal responds not to an actually nocuous 
stimulus but to some sign of approaching danger—to a shadow, the 
cracking of a twig, or to any object looming up and drawing near. So, 
too, the feeding response is often elicited not by direct contact with the 
food itself, but by a sign of it—its smell, its movement, the disturbance’ 
it makes. 
This leads us on to consider a point on which I touched before, namely, 
the nature of the perceptions, especially the visual ones, to which the 
animal gives significant responses. This is a field in which much 
interesting and important work has been done of late years. 
It has been shown in many cases that it is not the separate physico- 
chemical stimuli that are important in eliciting response, but the whole 
complex of stimuli taken together, their arrangement, their pattern, their 
relations to one another and to the visual field as a whole. A dog can 
recognise his master by sight, and it does not matter whether he sees him 
full face or in profile, standing up or sitting down, close at hand or a little 
way off. There is a general pattern or facies, with infinite variation in 
detail, to which essentially the response is made. He would be a bold 
man who would propose a connectionist or additive explanation of 
response to a varying and shifting pattern or image of this kind. 
Then there are the many examples known where response is made not 
to a particular visual datum per se but to it in its relations to other features 
in the perceptual field. ‘The simplest cases are those of ‘ relative choice,’ 
exemplified by K6hler’s experiments with chicks. He first of all trained 
them to respond to the darker of a pair of grey colours. He then sub- 
stituted a new pair of colours, consisting of the darker of the old pair 
and one still darker. He found that his chicks now reacted, not to the 
original grey, but to the darker of the new pair. They had really been 
trained to respond not to a particular shade of grey but to the darker of 
a pair. Many similar cases are known. 
Here is an observation by Bierens de Haan ® which shows in a striking 
manner how an animal may respond to an object only in its relation to 
other objects in the visual field. A young Pig-tailed Macacque was 
given the choice of two doors, one marked by a card with a red circle, 
while above the other was placed a card bearing a blue triangle. Food 
was placed behind the door with the red circle, and the monkey rapidly 
learned to choose that door. The experimenter then substituted for the 
blue triangle a blue circle or a red triangle, and he fully expected that the 
monkey would continue to choose the red circle. Instead of that the 
monkey was completely confused, and chose the red circle in only about 
fifty per cent. of the trials, When the blue triangle was restored, how- 
ever, it responded correctly and consistently. It appeared from these 
® Animal Psychology for Biologists, London, 1929, pp. 40-41. 
