“A striking example of two complicated sets of behaviour patterns evolving independently in unrelated species, yet in such a manner as to produce a great number of indubitable analogies is furnished by the behaviour of human beings and of geese when they fall in love and when they are jealous. Time and again I have been accused of uncritical anthropomorphism when describing, in some detail, this behaviour of birds and people. Psychologists have protested that it is misleading to use terms like falling in love, marrying or being jealous when speaking of animals. I shall proceed to justify the use of these purely functional concepts. In order to assess correctly the vast improbability of two complicated behaviour patterns in two unrelated species being similar to each other in so many independent points, one must envisage the complication of the underlying physiological organization. Consider the mini- mum degree of complication which even a man-made electronic model would have to possess in order to simulate, in the simplest possible manner, the behaviour patterns here under discussion. Imagine an apparatus, A, which is in communication with another one, B, and keeps on continuously checking whether apparatus B gets into communication with a third apparatus C, and which furthermore, on finding that this is indeed the case, does its utmost to interrupt this communication. If one tries to build models simulating these activities, for example in the manner in which Grey-Walter’s famous electronic tortoises are built, one soon realizes that the minimum complication of such a system far surpasses that of a mere eye.
The conclusion to be drawn from this reasoning is as simple as it is important. Since we know that the behaviour patterns of geese and men cannot possibly be homologous – the last common ancestors of birds and mammals were lowest reptiles with minute brains and certainly incapable of any com- plicated social behaviour – and since we know that the improbability of coincidental similarity can only be expressed in astronomical numbers, we know for certain that it was a more or less identical survival value which caused jealousy behaviour to evolve in birds as well as in man.”