234 THE PPxESEXT EVOLUTION OF MAN — PHYSICAL 



and telluric influences, and may prevail anywhere. Of 

 these diseases only syphilis need concern us, since it 

 alone is sufficiently prevalent to be a cause of evolution. 

 The second class includes those diseases of which 

 the pathogenic micro-organisms are entirely parasitic, 

 but are capable of maintaining existence for a limited 

 time, as resting spores or otherwise outside the living 

 tissues. Tuberculosis and the acute exanthemata are 

 examples. Normally they are always earth, air, or water- 

 borne, though they may be communicated by actual 

 contact, — e.g. by a kiss, — and therefore, since their 

 microbes are incapable of maintaining existence for an 

 unlimited time outside the living tissues, it follows that 

 they are essentially diseases of crowded populations. 

 In sparsely inhabited countries they must tend to die 

 out under normal circumstances from failure of their 

 food supply, and even when the infection is periodic- 

 ally renewed from outside, the inhabitants of such 

 territories must enjoy a greater freedom from infection 

 than the inhabitants of more populous lands where 

 these diseases are endemic. This freedom from infec- 

 tion, other things equal, will be greatest when the 

 country is inhabited by barbarous tribes hostile to one 

 another, as in Australia at the present day, or in North 

 America during the past, for this mutual hostility 

 gives rise to what is practically a very strict system of 

 quarantine, A very important point to be noted in 

 connection with disorders of this class is that, since the 

 microbes, before they can infect a fresh host, normally 

 must exist for a time outside the body, such diseases 

 cannot be prevalent unless the external environment 

 present conditions favourable to the existence of the 

 microbes as well as for their conveyance to other hosts, 

 conditions which are not always satisfied even in 

 populous countries. For instance, as regards tuber- 

 culosis, the conditions favourable to its prevalence are 



o 



