Primarily, the conquest of the Isthmian barrier was the conquest of the mosquito. Not mountains to be leveled, nor wild rivers to be tamed, nor yet titanic machinery to be installed, presented the gravest obstacles to the canal builders. Their most feared enemies were none of these, but the swarms of mosquitoes that bred in myriads in every lake, in every tiny pool, in every clump of weeds on the rain-soaked, steaming, tropical land. For these mosquitoes were the bearers of the dread germs of yellow fever and of malaria; and the conditions that encouraged their multiplication bred also typhoid and all manner of filthy disease. Each mosquito was a potential messenger of death. The buzzing, biting pests had defeated the French in Panama without the French ever having recognized the source of the attack. It was because the Americans, thanks to Great Britain and to Cuba, knew the deadly qualities of the mosquitoes that they were able to plan, under the leadership of Col. W. C. Gorgas, a sanitary campaign of unprecedented success. It achieved two victories. One was that it made of the Canal Zone the most healthful strip of land under tropic skies. The other is the Panama Canal. When one looks about in an effort to place the credit for these great sanitary achievements he must go back to Cuba, where the yellow fever commission, consisting of Reed, Carroll, Lazear, and Agrimonte, made the remarkable investigations proving that yellow fever is transmissible only through the bite of a mosquito. He must go still further back to Maj. Roland Ross of the British Army, and his epoch-making discovery that malaria is conveyed only by the bite of another kind of mosquito. And, if he is just to all who have contributed to the establishment of the insect-bearing theory of disease, he must not forget Sir Patrick Manson who first proved that any disease could be transmitted by insect bites. It was he who discovered that filariasis is transmissible by this method alone. It was from him that Ross gathered the inspiration that is releasing humanity from one of the most insidious of all the diseases to which mortal flesh is heir. And it was from Ross's malaria discoveries, in turn, that Reed carried forward to successful proof the theory which had persisted in some quarters for generations that yellow fever was transmissible through mosquitoes; a theory already partially proved by Dr. Carlos Finley, of Havana, 20 years earlier. [Illustration: COL. WILLIAM C. GORGAS THE HOSPITAL GROUNDS, ANCON] [Illustration: LIEUT. FREDERIC MEARS THE OLD PANAMA RAILROAD]