In the 19th century it was a generally accepted principle that the power of major organic chemical synthesis was confined to the plant kingdom and that animals could only oxidize, hydrolyze and make minor alterations to large molecules, which they ingested as such. Since iron was found to be in organic combination in blood, it seemed likely that natural diets would be found to supply it in essentially the same form. It was not unexpected, therefore, that from 1850 on into the next century large dietary supplements of iron salts were reported to be almost (or entirely) indigestible. Yet iron salts were undoubtedly effective in treating simple anemia. To escape from the paradox, it was argued that iron salts combined with hydrogen sulfide in the gut, thus sparing the natural organic iron of the diet and allowing it to be better absorbed. Despite experiments refuting this hypothesis, the belief that inorganic iron could not be used in hemoglobin synthesis lingered into the 1920s.