The power to assimilate crude inorganic matter as it is found in the soil, and convert it into living protoplasm and other organic substances, or to use such substances in performing physiological function, does not belong to the animal organism. It is the office of plant life or vegetation to convert the primary elements from their crude inorganic state into the organic state. This conversion cannot be accomplished by any synthetic process known to the laboratory. After the plant has raised the crude inorganic matter of the soil into plant protoplasm, the animal may take these and raise them to a still higher plane—that of animal protoplasm. But the animal cannot do the work of the plant. He must get his food either directly or indirectly from the plant kingdom. That is, the animal must either eat the plant or its fruits, or he must eat the animal that has eaten the plant. Food must be in the organic form. Air and water form the only exceptions to this rule.
Herbert M. Shelton, Food and Feeding
animals, diet, digestion, nutrition, plants