W. H. THORPE 79 



bird has learnt to follow models, the ability to transfer from one type to 

 another can be demonstrated in birds as young as 3 days and as old as 

 60 days; although when first presented with a new model they will show 

 a slight initial diminution in response, indicating that they perceive a 

 difference and that the new model is not at first quite as acceptable as 

 the old. With coot and moorhen, Hinde, Thorpe and Vince (1956) found 

 that in the early days of life the following-response is stronger than the 

 tendency (which is also present) to flee from strange objects. The result is 

 that, in the course of the following experiments, the younger bird gets 

 used to a number of strange objects and so its fleeing tendency is weakened 

 by habituation. But the bird's response is always ambivalent in some 

 degree and later the fleeing drive becomes stronger, and more difficult to 

 habituate; and consequently, with age, a strange object becomes less 

 likely to elicit following. In the coot, practically all the waning of a 

 following response with age can be attributed to this growth in the 

 fleeing drive and in these species no evidence was forthcoming for any 

 permanent effect able to influence instinctive behaviour patterns which 

 are to mature later. In some species, however, including some duck such 

 as the mallard (Weidmann, 1955, 1958) and also geese, there seems little 

 doubt that the tcrnunation of the imprinting period cannot be due solely 

 to the development of a competing fleeing drive but there that is an 

 internally controlled waning of the tendency to follow, independent of 

 ihe development of fear responses (jaynes, 1957). 



It is clear that, in the kind of experiment which I have just described, the 

 foUowing-reponse can be said in some way to be 'its own reward'. All 

 that the bird is doing in the first instance is giving rein to a need to 

 maintain itself by its own efforts in constant spatial relationship with a 

 moving object. This moving object can be almost anything which is not 

 too large or too small, and which does not move too fast or too slowly. 



To sum up this work on imprinting to visual stimuli, we can say 

 tentatively that the imprinting period is initiated primarily by the matura- 

 tion of an internally motivated, appetitive behaviour system which is in 

 readiness, at the time of hatching or very soon after, to express itself in the 

 following-response. The time during which this internally controlled 

 tendency can find its first expression in action is limited, to a matter of a 

 few hours or at most days, by internal factors; but during this time the 

 response is ready to appear as soon as the circumstances permit it to do so. 

 The termination of the ability to make that first response is, as we have 

 seen, very largely influenced by the development of competing fleeing 

 responses. Nevertheless, the evidence remains very strong that, in some 



