The problem of science will consist precisely in this, to seek the unitary character of physiological and pathological phenomena in the midst of the infinite variety of their particular manifestations.
--Claude Bernard (1865, p. 124 in Eng. Trans.)
The concept of a model seems to have preceded the frequent appearance of the term in biomedical research literature. In his classic work, An Introduction to the Study of Experimental Medicine, Bernard (1865) discussed "the usefulness to medicine of experiments on various species of animals." (See English edition, pp. 122–129.) Krogh (1929) stated, "For a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied." Almost half a century later, this became known as the August Krogh principle (Krebs, 1975). Krebs and Krebs (1980) cautioned that "an uncritical application of this principle may lead to fallacious generalizations, because extrapolating findings from one species to another is not invariably valid." Ross (1981) carried this point further, arguing that comparative physiology, rather than achieving its objectives of contributing to knowledge of phylogenetic relationships and of discovering the origins of physiological functions, has in reality dealt with the description of adaptations. On the other hand, Bullock (1984) argued that ''comparative neuroscience is likely to reach insights so novel as to constitute revolutions in understanding the structure, functions, ontogeny, and evolution of nervous systems." Without using the term, these authors were discussing what we refer to today as models.
The various kinds of models and their meanings were discussed by Ransom (1981), who wrote:
In its simplest form, a model is a simplified representation of a structure…. A heuristic model is a model used to discover how a process works rather than being a descriptive model of the process…. The definition of a