ABSTRACT
Three widely divergent views on the comparison of cognitive capacities, especially intelligence, are current. One is that these capacities develop, in evolution as in ontogeny, from very modest levels in primitive taxa to very high levels in some of the advanced taxa, though not in a linearly progressive or ladder-like way. A second applies to the vertebrates and asserts that evidence at hand does not justify recognizing any general difference in intelligence among the classes from fish to mammals or among the orders of mammals, except for the human species, which stands out from all others. The third view is that cognition and especially intelligence in different species is so different in quality that it cannot be compared in degree.
