The question naturally arises how far are we justified in drawing conclusions from the data furnished by these experiments on the point of whether this is a satisfactory regimen to which man may safely adhere throughout the adult period of life. It must be admitted that in the light of the new viewpoints which have been illuminated by animal experimentation during the past decade, these results on men do not seem sufficiently convincing to warrant any conclusions relative to the most important of all questions which arise in the life of the individual, that of maintaining to a late period in life the full vigor of youth.

These experiments on men covered approximately 1 per cent of the normal span of life for individuals of their age. It has been abundantly demonstrated on animals that diets which fall far below the standard of quality necessary to maintain life and vigor over a very great fraction of the span of life of which the species is capable of living, may cause no noticeable deterioration in the physical condition, during even 5 per cent of the normal expectation of life. In order to produce noticeable effects by faulty diet during 4 or 5 per cent of the average life, it is necessary that the faults shall be relatively severe. Yet diets which are just good enough to permit a young animal to develop in what appears to be an approximately normal manner, may cause early aging, and instability of the nervous system. Either during youth, while growth is still in progress, or after it has ceased, the adherence for even brief periods of a few days or weeks to a diet which is unsatisfactory in some degree, leads to a deviation from the normal histological structure of the osseous, the nervous or the circulatory tissues, depending upon the nature and extent of the faults in the food. Such abnormal modification of tissue structure, would ordinarily escape observation even to-day, and certainly a few years ago.