Taking the mammalia as a whole, there are two principal forms of digestion; the first is by means of the juices which the wall of the gut secretes, and which reduce the food to a form absorbable by the wall of the alimentary canal; the second is by means of organisms contained in the alimentary canal, which not only can perform a similar function, but which, by multiplying in the gut at the expense of the foodstuffs, can themselves form diet more nutritive than that in which they grow. In one way or another the organisms can convert materials in their food, which otherwise would be wasted, into articles of diet important to the animal in whose gut these lowly creatures thrive.