170 THE PRINCIPLES OF HEREDITY 



crowded that the microbes are able to pass from one 

 individual to another in unending succession. When the 

 succession fails the disease dies out and is not renewed again 

 except from foreign sources. Even in modern times microbic 

 disease is comparatively rare among nomadic tribes, and, 

 seemingly, was quite unknown in Arctic regions and in 

 many Pacific islands before its introduction by Europeans. 

 It follows that these maladies, in their modern well-defined 

 forms, must have made their appearance only after men had 

 peopled certain regions in considerable numbers. Bacteria, 

 which had previously subsisted on decaying organic material 

 or on lower animals, must then have changed their habits 

 and gradually adapted themselves to the new and increasing 

 supply of food. Since the microbic diseases of man are 

 generally quite distinct from those of lower animals, and 

 since probably they are more numerous, it is likely that in 

 the majority of instances the passage was directly from a 

 saprophytic life to an existence parasitic on man. The con- 

 ditions for the evolution, persistence, and spread of disease 

 are less favourable among lower animals. Men have not 

 only lived in vast and settled communities from remote 

 antiquity, but there has been more or less constant inter- 

 course between their different communities. Many lower 

 animals lead solitary lives. Many gregarious species wander 

 from place to place in search of food and leave the sick 

 behind to perish. Even when species are gregarious and 

 inhabit fixed dwelling-places the spread and persistence of 

 disease amongst them is checked by lack of intercourse 

 between the different communities of the same species. 

 Men, therefore, because they have presented a more constant 

 and abundant supply of food, have suffered probably more 

 from microbic disease than lower animals. 



288. We have historical evidence of the antiquity of many 

 diseases. Leprosy is mentioned in the book of Exodus as 

 prevalent in Egypt. By other accounts we learn that it 

 was prevalent in India, China, and Japan about the same 

 time. 1 The fact that it had spread so widely argues an im- 

 mensely greater antiquity. Venereal diseases are mentioned 

 by Greek and Roman writers, 2 as is diphtheria. 3 " Con- 

 sumption of the lungs may be traced with certainty in the 

 writings of every period as far back as the earliest attempts 

 of the ancient world to deal with medicine according to a 



1 Hirsch's Handbook of Geographical and Historical Pathology, vol. 

 ii., p. 2 (New Sydenham Society). 

 L a Op. cit., p. 60. 3 Op. cit., p. 73. 



