CAUSES OF INSALUBRITY 351 



and marshes arise, which become the great nurseries 

 of malaria. In the most flourishing days of Rome, 

 when the whole surface of the Campagna was populous 

 and in full cultivation, attention was alive to the im- 

 portance of drainage. Probably many of the swampy 

 tracts, whence mosquitoes now swarm, were then dry 

 and turned over by the plough or spade. The general 

 processes of agriculture prevented the accumulation 

 of stagnant water and rotting vegetation. But with 

 the fall of Rome and the devastation of the Campagna 

 by successive hordes of barbarians, the villas fell into 

 ruins, the inhabitants were in great measure extirpated, 

 the farms remained untilled, the soil was left untouched, 

 the water was allowed once more to gather and the 

 vegetation to decay in the hollows. In the well-known 

 words of Gibbon, ' the Campagna of Rome was 

 speedily reduced to the state of a dreary wilderness 

 in which the land is barren, the waters are impure 

 and the air is infectious.' Fever, which probably 

 always found a home in various parts of the district, 

 now stalked everywhere, until at the end of the twelfth 

 century, when the population of the city had fallen 

 to no more than 35,000 souls, Pope Innocent III. 

 could declare that it was difficult to find there a man 

 of forty years of age and hardly possible to meet with 

 one of sixty. 



No one will dispute that the sole, or at least the 

 chief, cause of this long-continued depopulation is to 

 be found in the prevalence of malarious fever. Nor, 

 since modern science has so clearly revealed the nature 

 and source of this decimating malady, can there be any 

 hesitation as to the more important steps that must be 



