We must now modify the definition of the saturation point, for, in addition to food supply, there are other factors which limit the maximum numbers of any species in a given area. There are many enemies to destroy them - indeed each species, man included, serves as food to some other. Pestilences due to microbic enemies are proofs that there are more men born than can survive. The reverse proposition is not true, though it is generally believed that large birth rates are necessary to repair the destruction of life from diseases. If populations are not dense, pestilenees are impossible, as a rule.
A certain tenuity of population of every animal species seems to be necessary for two reasons. In the first place, every species produces excreta which are poisonous to it, and which will kill it off if they are too concentrated. It is like certain low fermentative yeasts which produce alcohol and which will cease their activity as soon as the percentage of alcohol mounts to a given point. In the next place, crowding gives a chance for the spread of fatal parasitic diseases from individual to individual. As a rule, crowding is followed by diseases reducing the numbers to the saturation point, which we thus see is not exactly the number which can be fed, but the number able also to escape enemies. For instance, coffee trees were introduced into Ceylon, and also into Batangas, P. I., and bid fair to make a permanent success, when the "blight" (a fungus) killed them all off. In Ceylon, it has been found that a few plantations can live, and they are now springing up again, but widely separated and thinner.
Applying these rules to man we find that density of population is strictly dependent upon sanitation. The more dense the population the more elaborate and expensive must be the means of removing our own poisons, or epidemics will thin us out to the proper tenuity. Farmers' boys may be perfectly healthy in the crude sanitation of the farm, but if they cluster together in a camp, and try the same methods, typhoid will wipe them out of existence. Our recent army experiences are too fresh in mind to need recalling. The impossibility of crowding savages, and their great death rate - both due to filthy habits - are mentioned by H. G. Wells* "The real savage is a nest of parasites within and without; he smells, he rots, he starves. Forty is a great age for him".
Instinct is still against vaccination and sanitation, for man's nature is a result of evolution in an isolated state, and he does not yet know how to live in communities. He began crowding into towns and villages long before he knew the results of overcrowding, and there has always been a frightful mortality from crowd diseases, which have become our new enemies in place of the adverse conditions of old. Though savage life is an extremely filthy one, the men are so isolated and the poison is so diluted that it is harmless as compared with modern conditions.