The most elaborate attempt to test the relative merits of the strictly vegetarian diet as contrasted with the omnivorous type was made by Slonaker (4). He fed a group of young rats on a list of 23 vegetable foods, allowing them free choice within limits. For comparison a similar group were fed the same foods of vegetable origin, but in addition animal food was given in moderate quantities. Since several natural foods, raw or prepared, were offered at a time, and the animals were allowed free choice as to what they should eat, and since no effort was made to keep a record of food consumption or of the relative amounts of the different foods eaten, the results cannot be used for critical examination except in a limited way. They are, however, of the utmost interest in showing how far instinct fails to guide an animal in the selection of food. Slonaker's list of foods included nearly everything which a vegetarian in Southern California would be likely to have on his table and included seeds, the milling products of seeds, leafy vegetables, tubers and fleshy roots.

The vegetarian group grew fairly well for a time, but became stunted when they reached a weight of about 60 per cent of the normal adult size. They never increased in weight beyond this point. The omnivorous controls grew steadily to what may be regarded as the normal size for the adult. The vegetarians lived, on the average for the entire group, 555 days, whereas the om-nivora had an average span of life of 1,020 days. The vegetarian rats, in other words, grew to be about half as large and lived half as long as did their fellows receiving animal food. Slonaker drew the conclusion that a strictly vegetarian diet was not suitable for the nourishment of an omnivorous animal, but was not able to give the cause of the deficiency.