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It has often been assumed that animals given a suitable free choice of diets are able to select satisfactorily according to their individual nutritional needs. However, no systematic work has hitherto been undertaken to ascertain how far this is true, nor have we any knowledge of the mechanism involved in the choice. Lusk (1928), in his well-known text book on nutrition, refers to the observation of a German worker (Tscherkes, 1923) that fowls suffering from polyneuritis will search out green food and refuse to accept grain. This be describes as due to the "triumph of instinct." The similar eagerness of vitamin B deficient rats to consume diets containing the vitamin must, we imagine, have been noticed by observant workers. It is, however, unsatisfactory to allow the matter to be ascribed vaguely to some unexplained—it nor inexplicable—" instinct." It animals are in tact able to distinguish between foods containing and deficient in the vitamin, what is the nature of the process involved? Has the animal some means, as by taste or smell, of recognizing the vitaminper se ? It so, it would imply an ability to detect a constituent amounting to no more than perhaps 1 part in a million of the food. It nor, by what means is the rat able to recognize the vitamin-containing food ? And what are the limitations to such powers of recognition ? These are some of the questions which one sought to answer, An effort to analyse these phenomena was begun in this laboratory in 1928, and in the present paper are summarized the principal results reached during the period 1928-1931.* To anticipate a main conclusion it max be said at once that we have obtained good evidence that the behaviour of the animal is due not so much to instinct as toexperience, i. e ., of the beneficial effect produced by a particular foodstuff. We believe that This factor of experience plays an important part in determining dietary preferences in general, and it will certainly have to be taken into account in future work on that hitherto neglected subject, the psychology of appetite. Apart from the reference quoted above we know of no previous literature relating to the original point of departure of our enquiries. Recorded work on the whole question of the free choice of diet is almost equally scanty. Osborne and Mendel (1918), working on the comparative food values of different proteins, thought that rats " as a rule ate more of the adequate than of the inferior food " ; but Beadles, Braman and Mitchell (1930), on the contrary, could find " no support for the assumption that the more complete of the two rations is consumed in greater amount." Nevens (1928) noted that cattle given a free choice of limestone, bone meal and salt, offered as supplements to an inadequate diet, took only insignificant amounts of the first two ; while kon (1931) found that rats might fail to take enough protein even to keep them alive, when allowed to choose their own allowance of carbohydrate, protein and salt mixture.

Appetite and choice of diet. The ability of the vitamin B deficient rat to discriminate between diets containing and lacking the vitamin