Here the trails of the extermination party fork, one of them taking the perfectly obvious but rather troublesome direction of protecting houses and particularly bedrooms with suitable screens and keeping the inhabitants safely behind them from about an hour before sundown on. By this simple method alone, parties of explorers, of campers, of railroad-builders going through swamps, of the laborers on our Panama Canal, have been enabled to live for weeks and months in the most malarious regions with perfect impunity, so long as these precautions were strictly observed. The first experiment of this sort was carried out by Bignami upon a group of laborers in the famous, or rather infamous, Roman Campagna, whose deadly malarial fevers have a classic reputation, and has achieved its latest triumphs in the superb success of Colonel Gorgas at Panama. While this procedure should never be neglected, it is obvious that it involves a good deal of irksome confinement and interferes with freedom of movement, and it will probably be carried out completely only under military or official discipline, or in tropical regions where the risks are so great that its observance is literally a matter of life or death. The other division of malaria-hunters pursued the trail of the _Anopheles_ to her lair. There they discovered facts which give us practically the whip-hand over malarial and other tropical fevers whenever we choose to exercise it. It had long been known that the breeding-place of mosquitoes was in water; that their eggs when deposited in water floated upon the surface like tiny boats, usually glued together into a raft; that they then turned into larvae, of which the well-known "wigglers" in the water-butt or the rain-barrel are familiar examples; and that they finally hatched into the complete insect and rose into the air.