134 POPULAR SCIENCE MONTHLY. 



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 compara- 

 tively 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 cen- 

 tury (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 drinlving water. Some years 

 later a similar conviction was reached regarding typhoid fever. 



The medieval ignorance concerning the direct infectivity of drink- 

 ing water and its importance as a factor in the spread of disease told 

 heavily against the cities. In sparsely populated districts the likeli- 

 hood tliat 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 



* A table is given by Kuczynski which shows the relative shares of immi- 

 gration and excess of birth rate in producing the growth of Berlin. 



