THE DISEASES OF THE LOWER ANIMALS 275 



not be contracted in the open air. Its microbes are found especi- 

 ally in such dark, ill-ventilated, and crowded dwellings as are 

 built by the civilized inhabitants of cold and temperate regions. 

 More especially it affects the larger towns and cities, and yet 

 more the slums of these. Beyond all other complaints it is the 

 disease of civilization. As in the case of syphilis, its long 

 duration confers considerable powers of journeying to distant 

 lands. 



457. For all we know or can surmise to the contrary, distinctively 

 human contagious and insect-borne maladies may have prevailed 

 during the early Stone Age. Air-borne maladies can hardly have 

 come into being till large continental tracts had been relatively 

 densely peopled by hunters and nomads. The advent of water- and 

 earth-borne diseases must have occurred later. Their existence 

 implies large and settled populations. In the one case the water- 

 supply, and in the other stationary dwellings must have been 

 constantly used and infected. Probably, of all animals, human 

 beings suffer most, or at least from the greatest number of microbic 

 diseases. More than any other animal, man now dwells in large 

 and settled communities, between which there is constant inter- 

 course. Many insect-borne diseases of the lower animals, however, 

 are known to be endemic in certain regions, as for example the 

 Texas fever of cattle, which is carried by ticks, and the fly-disease 

 in South Africa, which is borne by the tsetse fly. Races long 

 afflicted by them tend to evolve resisting power, whilst, as in the 

 case of man, immigrants suffer severely. Air-borne diseases, such 

 as are perhaps the rabbit plague of Canada and rinderpest which 

 lately more than decimated the herbivora of Africa, are the causes 

 of the great murrains or pestilences which tend to appear whenever 

 animal populations become dense. The distemper of dogs appears 

 to have become endemic amongst the large and settled canine 

 population supported by man, but amongst wild animals air-borne 

 diseases appear always to be epidemic. Earth-borne diseases occur 

 especially amongst browsing animals, which acquire them through 

 herbage that has been tainted by infected individuals. Probably 

 the disease which destroys hive - bees is earth - borne. Since 

 recovery within definite time from microbic diseases implies 

 immunity, and since the vast destruction of life greatly diminishes 

 the available supply of nutriment for the microbes, epidemics 

 amongst lower animals are usually very transient. Probably the 

 starting-point of every great pandemic of earth-borne disease is 

 some area where the animal hosts, having undergone evolution, are 



