Page:Popular Science Monthly Volume 63.djvu/138

134 population than actually occurred, and this transfer merely sufficed to keep the city population at a fairly constant level. As soon, however, as the city death rate began to decline and even to fall below the birth rate, the city population increased with leaps and bounds. This change is comparatively modern. London did not show a natural increase, due to excess of births, until the beginning of the nineteenth century, and Berlin did not reach this point until 1810.

It must not be forgotten, moreover, that simple excess of birth rate is not a fair measure of the decline that has occurred in the death rate. The birth rate itself has not remained constant, but in the last thirty years has materially diminished in nearly all civilized lands, so that in reality the decline in death rate is far greater than can be indicated by mere change in the absolute or proportional excess of births.

If the large cities have lost some of their former evil repute in the matter of healthfulness, the improvement must plainly be attributed to the development of the art of municipal hygiene. The dangers to health resulting from the massing of human beings within comparatively narrow limits are now fairly well known, but such knowledge has not always been available and is even now not always acted upon. The question of water supply affords a pregnant illustration. That some connection existed between outbreaks of disease and the character of drinking water was seen darkly all through the middle ages, but the groping speculations on the subject only led to the hypothesis, fraught with terrible consequences to an unhappy people, that 'the Jews had poisoned the wells.' It was not until about the middle of the last century (1854) that an explosion of cholera in London among the users of water from the 'Broad Street Pump' established definitely in the minds of physicians the truth that the specific poison of Asiatic cholera could be conveyed by means of infected drinking water. Some years later a similar conviction was reached regarding typhoid fever.

The medieval ignorance concerning the direct infectivity of drinking water and its importance as a factor in the spread of disease told heavily against the cities. In sparsely populated districts the likelihood that any particular well or spring would become infected was comparatively slight, and even if a single well did become accidentally polluted neighboring wells or springs used by other families might still