There is reciprocity between the teeth and digestive organs of animals and their natural food.  The grasses, leaves, &c., which are consumed by the herbivora, contain a large proportion of cellulose and woody tissue.  Consequently, the food is bulky; it is but slowly disintegrated and the nutritious matter liberated and digested.  The cellulose appears but slightly acted upon by the digestive juices.  The herbivora possess capacious stomachs and the intestines are very long.  The carnivora have simpler digestive organs and short intestines.  Even they consume substances which leave much indigestible residue, such as skin, ligaments and bones, but civilised man, when living on a flesh dietary removes as much of such things as possible.  The monkeys, apes, and man (comprised in the order Primates) have a digestive canal intermediate in complexity and in length to the herbivora and carnivora.  A certain quantity of indigestible matter is necessary for exciting peristaltic action of the bowels.  The carnivora with their short intestinal canal need the least, the frugivora more, and the herbivora a much larger quantity.  The consumption by man of what is commonly called concentrated food is the cause of the constipation to which flesh-eating nations are subject.  Most of the pills and other nostrums which are used in enormous quantities contain aloes or other drugs which stimulate the action of the intestines.

Highly manufactured foods, from which as much as possible of the non-nutritious matter has been removed is often advocated, generally by those interested in its sale.  Such food would be advantageous only if it were possible to remove or modify a great part of our digestive canal (we are omitting from consideration certain diseased conditions, when such foods may be useful).  The eminent physiologist and bacteriologist, Elie Metchnikoff, has given it as his opinion that much of man’s digestive organs is not only useless but often productive of derangement and disease.  In several cases where it has been necessary, in consequence of serious disease, to remove the entire stomach or a large part of the intestines, the digestive functions have been perfectly performed.  It is not that our organs are at fault, but our habits of life differ from that of our progenitors.  In past times, when a simple dietary in which flesh food formed little or no part, and to-day, in those countries where one wholly or nearly all derived from vegetable sources and simply prepared is the rule, diseases of the digestive organs are rare.  The Englishman going to a tropical country and partaking largely of flesh and alcohol, suffers from disease of the liver and other organs, to which the natives and the few of his own countrymen, living in accordance with natural laws are strangers.

Indigestible Matter—­Food is never entirely digested.  As a reason against confining ourselves solely to vegetable food, it has been stated that such is less perfectly digested than animal food and that it therefore throws more work on the digestive organs.  It is also urged that on this account a greater quantity of vegetable food is required.  We have shown elsewhere that, on the contrary, vegetarians are satisfied with a smaller amount of food.  Man requires a small quantity of woody fibre or cellulose in his food to stimulate intestinal action and prevent constipation.