It is therefore apparent that epidemic pneumonic plague is controllable by sanitary and hygienic measures and, furthermore, that in the absence of original cases of bubonic and septicæmic plague, with secondary plague pneumonias which give rise to primary plague pneumonia in the manner explained, respiratory plague in epidemic form will not occur. There is no evidence pointing to the conveyance of respiratory plague by insects or other carriers. CHAPTER III ITS CONTROL AND SUPPRESSION PLAGUE PREVENTION.--At present the most promising and the most rationally based phase of plague control is that of prevention. The reason for this is plainly apparent. If the facts in the case are as stated and if the conclusions of the Plague Commissioners and students of epidemiology the world over are correct, to eradicate plague we need only to control its carriers. To exterminate the rat (and perhaps the marmot and ground squirrel), to prevent the transportation of rats or of infected rat fleas in ships, trains, clothing, merchandise and upon the bodies of men and animals from the numerous foci or plague centres of the world to non-infected localities, is a beautiful plan indeed. Restricted to single communities, even where the intelligence, patriotism, effort and wealth of the whole people are enlisted, the undertaking is formidable, with obstacles to its execution, and discouragement must often be expected. Extended in its application to the whole plague-infected world it becomes an undertaking seemingly impossible of accomplishment. Yet we are encouraged to face the situation by a glance at what has been accomplished. The United States, perhaps, presents the highest examples of achievement in the cases of San Francisco and Manila. The work in San Francisco is too recent and has been too well published to require detailed review here. A successful campaign against rats in 1907 practically terminated an epidemic of considerable proportions well within a year. Behind this movement, however, were the powerful machinery of the Federal Government, money in generous amount and a considerably aroused public, resentful of the mismanagement of the 1903 epidemic, whereby, through pure fear of financial loss to commercial interests and by a disgraceful suppression of the truth, California was made, permanently perhaps, one of the world's plague centres. It has been estimated that the rat population of the world is equal to the human population, and this estimate does not appear to be unreasonable when one considers as indices the destruction of the rodents in cities by the hundreds of thousands, upon single farms by the thousand, and the wonderful procreative powers of the rat. ECONOMIC IMPORTANCE OF RAT DESTRUCTION.--It is certain that the economic importance of rat destruction upon grounds other than those purely sanitary must be impressed upon the public wherever a rat campaign is to be carried on.