Under this heading we will consider the chemical changes which take place in the human alimentary canal, which are not beneficial or necessary to normal digestion. The cause of the most important abnormal changes in the contents of the stomach and the intestines is the presence of living micro-organisms called bacteria.

In the lesson entitled "Evolution of Man," a general survey of the history of man's development from lower forms of life is given. In this general work I do not elaborate extensively upon the method by which evolution proceeds, but those who are acquainted with the writings of Darwin, and other evolutionists, are familiar with the phrases "the survival of the fittest," and "the struggle for existence."

Bacteria chief causes of abnormal changes.

As we commonly think of "the survival of the fitteest" in animal life, we picture the death-struggle of the captured animal, or the fight for food in times of scarcity, or, if it be in the case of plants, the crowding or the struggling for soil and sunlight. We can apply the same principle to bacteria and to other microscopic forms of life.

Bacteria, while minute masses of unconscious protoplasm, are, by the laws of growth and reproduction, struggling for existence just as truly as are the more conspicuous forms of life.

Because of the invariable presence of greater or less quantities of bacteria within the intestines of all ordinary animals, some scientists insist that their presence is in some way necessarily related to the life of the animal, and is probably beneficial.

"Survival of the fittest" among bacteria.

New-born animals, however, are free from bacteria, and the bacterial germs found in the more matured animal must, therefore, have been taken into the alimentary canal with food. Ingenious scientists have taken new-born guinea pigs, and have kept them in sterile or germ-proof compartments, giving them filtered air to breathe, and absolutely sterile food. These pigs lived and thrived through the experiment as did their fellows outside the bacterial-proof dwelling. This is considered good evidence that bacteria accumulate in the digestive organs of all animals, not for a purpose connected with animal physiology, but because in order to digest and to assimilate food, conditions are established which are so nearly like those required for bacterial growth, that bacteria are produced, or take advantage of the favorable conditions, just as weeds, if given a chance, thrive in a cultivated field.

Experiments proving accumulation of bacteria.

I have already referred to the antiseptic or germ-destroying properties of the gastric juice, and to other secretions of the digestive organs. This would suggest that the growth of bacteria is undesirable from the standpoint of man's welfare. There are many species of bacteria growing in the human intestines, hence we cannot say with certainty that all this bacterial growth is harmful, as, in order to determine this, the resulting waste-products of each particular species of bacteria would need to be considered separately. We can, however, make the general statement that bacteria are abnormal, or foreign to the human digestive canal, and that their presence is detrimental to human welfare.

Micro-organisms give off various substances as waste-products of their growth, dependent upon the species of bacteria, and the material in which they are growing. Thus the waste-products of the yeast-plant are carbon dioxid and alcohol. In the alimentary canal there exists an abundance of carbohydrate and proteid substances which form excellent food material for numerous species of bacteria. The substances produced by the growth of these various kinds of bacteria are numerous. They include the gases, carbon dioxid, hydrogen, hydrogen sulfid, marsh-gas or methane, and ammonia. Butyric, lactic, and other acids, together with alcohol, are also produced as a product of bacterial fermentation in the intestines. Perhaps the most detrimental of all are the substances produced by the bacterial putrefaction of proteids, of which in-dol and skatol are the two most important. Under ordinary conditions the bacteria themselves do not penetrate the intestinal walls, and their evil influence would be confined to mechanical disturbance of gas in the digestive organs, and to the destruction of a portion of the nutritive material of food, were it not for the fact that these harmful and poisonous waste-products I have mentioned, are soluble, and hence pass through the intestinal walls with the digested food material, into the blood, and are thus distributed throughout the body. It has been observed in the presence of intestinal congestion, where the food lies in the intestines for an abnormally long period, that the amount of these harmful nitrogenous decomposition products excreted by the kidneys, is considerably increased, proving that these products have circulated throughout the body.

Not all bacterial growth is harmful.

Waste-products of bacterial fermentation.

Arterio-sclerosis, or the hardening of the walls of the arteries, which has for many years been recognized by scientists as one of the principal causes of old age, comes from two causes:

Solubility and distribution of bacterial waste-products.