The conditions which have chiefly fostered it are the immigration of people who are accustomed to community life, the increase in factory life and the increased number of people of wealth who seek the advantages which the city gives them.  The city has always been the favored playground for the social game.  The unhealthy conditions of city life are due to the crowding, the more uncertain means of livelihood, the greater influence of vice and alcoholism.  Prostitution and the sexual diseases are almost the prerogatives of the cities.

3.  All means of transportation have increased and communication between peoples has become more extended and more rapid.  In the past isolation was one of the safeguards of the people against disease.  With the increase and greater rapidity of communication there is a tendency not only to loss of individuality in nations as expressed in dress, customs, traditions and beliefs, but many diseases are no longer so strictly local as formerly—­pellagra, for example.  Only those diseases which are transmitted by insects which have a strictly local habitat remain endemic, although the region of endemic prevalence may become greatly extended, as is seen in the distribution of sleeping sickness.  Diseases of plants and of animals have become disseminated.  Any plants desirable for economic use or for beauty of foliage and flower become generally distributed, their parasites are removed from the regions where harmonious parasitic inter-relations have been established, and in new regions the parasites may not find the former restrictions to their growth.  There have been many examples of this, such as the ravages of the brown-tail and gypsy moths which were introduced into New England and of the San Jose scale which was introduced into California.  There have been many other examples of the almost incredible power of multiplication of an animal or plant when taken into a new environment, removed from conditions which held it in check, as the introduction of the mongoose into Jamaica, the rabbit into Australia, the thistle into New South Wales and the water-plant chara into England.

It is very difficult to say, but it seems as though there is an increasing unevenness in the distribution of wealth, an increase in the number of persons who live at the expense of the laboring class.  Mass labor, effective though it be, makes it easier to divert the proceeds of labor from the laborers.  The evidence of this is seen in the increase in number and the prosperity of those pursuits which purvey to luxury, as the automobile industry and the florists’ trade and the greatly increased scope and activity of the social game.  On the other hand, there is an increase in the number of people who are to a greater or less extent dependent upon extraneous aid, evinced among other ways by the increase in the asylum populations.  Both these conditions, wealth and poverty, are important disease factors.  Tuberculosis is now a disease of the proletariat chiefly.  The