Page:The story of milk.djvu/170

 growth of the young, which our present knowledge of chemistry cannot explain. In Bulletin No. 17 of the Wisconsin Experiment Station experiments with the feeding of rats are described which show how butter-fat could not be replaced in the ration by vegetable fats of apparently the same composition and digestibility without disastrous results, and similar conditions have been found in regard to other foodstuffs, proteins as well as fats. The yolk of eggs and butter-fat contain this unknown something which is absolutely essential for the growth of the child and which is missing in most substitutes, especially in lard and vegetable fats.

The rat on the left got five per cent of cottonseed oil and the one on the right got instead one and a half per cent of butterfat, otherwise their rations were alike. These results are typical for any ration made up of purified foodstuffs with butterfat in them as compared with any fat of plant origin. The plant fats lack an unknown something without which growth cannot proceed.

The above illustration is from the work of McCollum and Davis at the Wisconsin Experiment Station.

Realizing the fallacy of the old rules for making up rations for the feeding of farm animals, Professor Evvard of Iowa is trying the reliability of the instincts of animals as a guide to the proper selection of the most