One consequence of this string of beliefs is that most attempts to study animal intelligence have consisted simply of a search for the precursors of human intelligence. Even Darwin himself, in The Descent of Man (1871), was concerned largely to find elements of human intelligence in other animals - of human language in bird song or the vocal imitations of parrots, and of human conscience in the behaviour of a dog towards its master. Darwin at least saw that it was important to decompose human intelligence into component processes or operations. In the hands of his successors, however, the enterprise rapidly reduced to the compilation of anecdotes designed to illustrate, in a wholly unanalysed way, how clever or human-like animals were.

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We all believe that we are more intelligent than other animals, and most of us believe that evolution has involved a progressive increase in intelligence as we ascend the phylogenetic scale, from invertebrates to lower vertebrates, to mammals, primates, our cousins the great apes, and at last ourselves.

I want to argue that this is an almost wholly erroneous view of the evolution of intelligence; that there is no single thing called intelligence; that we certainly cannot rank-order animals by their possession of it; and even more certainly that there is no linear progression from lower to higher vertebrates, lower to higher mammals, and then onwards and upwards to monkeys, apes and humans.

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A further source of temptation here is that we have a single word, 'intelligence' for what we are talking about, as though there were a single thing or unitary trait for that word to refer to. If intelligence is unitary, it follows that we might be able to rank-order animals by how much they had of it, just as IQ testers are inclined to rank-order people by the amount of intelligence they are supposed to possess.

Intelligence is much more profitably viewed as a diverse or heterogeneous array of processes, operations and skills. Some of these are general and surprisingly widespread - being common, for example, to essentially all vertebrates; some are specialised for particular purposes and tasks and may be much less evenly distributed. But there is surprisingly little evidence to support the view that more advanced forms of intelligence are to be found only in higher animals.

Most of us seem to find it perfectly reasonable to be asked whether a monkey is more intelligent than a bird. People tend to agree in the answers they give to such questions, ranking apes above monkeys, and monkeys above dogs and cats - though there is some dispute about which of those two is cleverer. Cats and dogs are seen as cleverer than horses; horses, in turn, are cleverer than cows and sheep, which are certainly more intelligent than chickens, which are probably more intelligent than fish.