tropics, hardened to the insect plagues of Guiana and the Orinoco.  But they reported that never had they been so tortured as in the Chaco.  The sand-flies crawled through the meshes in the mosquito-nets, and forbade them to sleep; if in their sleep a knee touched the net the mosquitoes fell on it so that it looked as if riddled by birdshot; and the nights were a torment, although they had done well in their work, collecting some two hundred and fifty specimens of birds and mammals.

Nevertheless for some as yet inscrutable reason the river served as a barrier to certain insects which are menaces to the cattlemen.  With me on the gunboat was an old Western friend, Tex Rickard, of the Panhandle and Alaska and various places in between.  He now has a large tract of land and some thirty-five thousand head of cattle in the Chaco, opposite Concepcion, at which city he was to stop.  He told me that horses did not do well in the Chaco but that cattle throve, and that while ticks swarmed on the east bank of the great river, they would not live on the west bank.  Again and again he had crossed herds of cattle which were covered with the loathsome bloodsuckers; and in a couple of months every tick would be dead.  The worst animal foes of man, indeed the only dangerous foes, are insects; and this is especially true in the tropics.  Fortunately, exactly as certain differences too minute for us as yet to explain render some insects deadly to man or domestic animals, while closely allied forms are harmless, so, for other reasons, which also we are not as yet able to fathom, these insects are for the most part strictly limited by geographical and other considerations.  The war against what Sir Harry Johnston calls the really material devil, the devil of evil wild nature in the tropics, has been waged with marked success only during the last two decades.  The men, in the United States, in England, France, Germany, Italy—­the men like Doctor Cruz in Rio Janeiro and Doctor Vital Brazil in Sao Paulo—­who work experimentally within and without the laboratory in their warfare against the disease and death bearing insects and microbes, are the true leaders in the fight to make the tropics the home of civilized man.

Late on the evening of the second day of our trip, just before midnight, we reached Concepcion.  On this day, when we stopped for wood or to get provisions—­at picturesque places, where the women from rough mud and thatched cabins were washing clothes in the river, or where ragged horsemen stood gazing at us from the bank, or where dark, well-dressed ranchmen stood in front of red-roofed houses—­we caught many fish.  They belonged to one of the most formidable genera of fish in the world, the piranha or cannibal fish, the fish that eats men when it can get the chance.  Farther north there are species of small piranha that go in schools.  At this point on the Paraguay the piranha do not seem to go in regular schools, but they swarm in all the waters and attain a length of