This statement of the deductions which it seemed justifiable to draw from the experimental evidence at hand in 1915 can be applied to-day to the nutrition of the rat. It has, however, been found that there exists a third dietary essential which appears not to be necessary in the food of the rat, but is indispensable in the diet of man, monkey and the guinea pig. This is a substance which protects against the syndrome of scurvy. It will be discussed at length in Chapter VIII (The Dietary Deficiency Diseases Scurvy. 178. The Relation Of The Diet To Certain Diseases Has Long Been Suspected).

From what has been said of the experimental studies which ultimately led to the conception that there were necessary two uncharacterized dietary essentials in addition to the long recognized food principles in the diet of the rat, it will be appreciated that about 1914-15 there were suggestions of various kinds "in the air," and that the time had arrived when some one of the several investigators working in the field would soon formulate on the basis of definite experimental evidence a working hypothesis concerning the essentials of an adequate diet. Thus in November, 1914, Mendel, in his Harvey lecture, said, "It is not unlikely - to speak conservatively - that there are at least two 'determinants' in the nutrition of growth. One of these is furnished by our 'protein-free milk,' which insures proper maintenance even in the absence of growth. . . . Without this 'determinant' . . . the special components of butter fat or cod liver oil or egg fat induce only limited gains at best. Another 'determinant' is furnished by these natural fats. Either of the determinants may become 'curative'; both are essential for growth when the body's store of them (if such there be) becomes depleted. It is too early to attempt a tenable conclusion" (11).

40. Stepp Failed To Interpret Correctly His Results

Stepp was unable to detect any special dietary property in butter fat, while he could easily do so when alcoholic extracts of certain natural foods were used. This is now readily understandable. His alcoholic extracts contained some of both the uncharacterized substances discussed above, whereas butter fat contains but one. The latter, without a supplementary source of the second one of these dietary factors would permit of failure of the experimental animals.