rers 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.

Obviously, there were two points at which the destroyers might strike,
the egg and the larvae. It was first found that, while the eggs required
no air for their development, the larvae wiggled up to the surface and
inhaled it through curious little tubes developed for this purpose,
oddly enough from their tail-ends. If some kind of film could be spread
over the surface of the water, through which the larvae could not obtain
air, they would suffocate. The well-known property of oil in "scumming
over" water was recalled, two or three stagnant pools were treated with
it, and to the delight of the experimenters, not a single larva was able
to develop under the circumstances. Here was insecticide number one. The
cheapest of oils, crude petroleum, if applied to the pool or marsh in
which mosquitoes breed, will almost completely exterminate them. Scores
of regions and areas to-day, which were once almost uninhabitable on
account of the plague of mosquitoes, are now nearly completely free from
these pe