The most natural application of this doctrine would be the use of the organs of the lower animals for the treatment of the corresponding diseased organs in man.

Thus diseases of the heart were to be treated with the hearts of animals, liver disorders with livers, and so on.

But this apparently simple form of treatment had endless modifications and restrictions, for not all animals were useful.

For example, it was useless to give the stomach of an ox in gastric diseases when the indication in such cases was really for the stomach of a rat.

Nor were the organs of animals the only "signatures" in nature.