The filthiness of all lower classes of civilized men, and of barbarians and savages is not conceivable to one who has not investigated. If savages cluster in groups as dense as barbarians, they die of crowd disease - plagues or epidemics. Barbarous life is a less filthy one, but the men are still isolated, and the poisons so diluted as to be harmless, yet if they cluster into the groups of civilized man, they die off. As man's rate of increase has kept him crowded to a degree beyond his ability to escape his own poisons, we find that history is a long record of the epidemics which have thinned out the filthy populations, a tenuity wherein contagion was less easily carried or their poisons were too dilute to be harmful.
* Cosmopolitan, November, 1902.
For instance, England, in 1348, had but 3,000,000 or 4,000,000 people, and was so frightfully dirty, self-poisoned and overcrowded, that over half were killed by the plague of that year. Previous to Jenner's time, according to the estimates of Ber-nouilli, the mathematician, 15,000,000 people died in Europe every twenty-five years from smallpox. It caused ten per cent, of all deaths, and half the deaths of children less than ten. Macauley states that in London before vaccination times, it was rare to see an adult unmarked by smallpox. The destruction of life was tremendous, as one-fourth of mankind was thus killed or crippled.
It is believed that every microbic disease afflicting man has been evolved through his filthy habits in overcrowded communities. Originally, the germs were all harmless saprophytes occupied in destroying dead organic matter. As filth collected around the habitations, those species of bacteria survived in greatest numbers which were carried to the filth by man himself, and those species were carried best which, by purely accidental variations, originating in some unknown way, were able to live in man for a while. Then those survived in the largest number, which by accidental variation were able to live the longest in man, even if by this parasitic existence they occasionally killed him. Thus typhoid fever is probably a very recent disease, speaking geologically; that is, the ancestor of the bacillus typhosus was probably a harmless saprophyte in the post glacial period. But such changes in species take immense time, we cannot bring them about in our laboratories. We can attenuate bacteria by changing their environment, but they revert to ancestral types as soon as conditions are restored. These speculations are mentioned merely to show the increased certainty that man has always been filthy from overcrowding, owing to a birth rate too large for his condition of culture.
Dr. G. Archdall Reid* shows the impossibility of savage or barbarous people clustering in dense masses, since they cannot resist infection. There has to be a gradual growth of immunity by killing off the most susceptible through many centuries as with our ancestors, who thus were weeded out by measles, etc. The susceptible died and the resistant lived - and though we contract such diseases now as ever, yet few die. When introduced among savages, measles kills them like a plague, as they have never evolved immunity. He says that for this reason savages are little capable of "achieving civilization," and our civilization is conditioned by our power of resisting certain infectious diseases. Robert L. Stevenson* mentions a tribe wiped out by smallpox and consumption. Reid mentions a race (Boggara) compelled to live on the desert, scattered, with no trees or water, who can live only in this way, because highly susceptible to all kinds of infectious diseases.