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 waterborne, 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 periodically 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 infection, 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 tuberculosis, the conditions favourable to its prevalence are
