In this variety of battle, airplanes and observation balloons play a prominent part. Once the enemy is driven out of its trenches, the message is flashed by wireless to the artillery and slaughter at long range begins. If there have been no intrenchments, as was the case in the first battle of the Marne, massed artillery send a plunging fire into the columns moving in open order and prepare the way for machine gunners and infantry to finish the rout. In previous wars, cavalry played a heroic role in open warfare; only rarely has it been possible to use cavalry in the Great War. The Germans sent a screen of Uhlans before its advancing hordes into Belgium and Northern France in 1914. The Uhlans also were in the van in the Russian invasion, but with these exceptions, German cavalry was a negligible factor. British and French cavalry were active in pursuit of the fleeing Teutons when the Hindenburg line was smashed in September of 1918. Outside of that brief episode, the cavalry did comparatively nothing so far as the Allies were concerned. It was the practice on both sides to dismount cavalry and convert it into some form of trench service. Trench mortar companies, bombing squads, and other specialty groups were organized from among the cavalrymen. Of course the fighting in the open stretches of Mesopotamia, South Africa and Russia involved the use of great bodies of cavalry. The trend of modern warfare, however, is to equip the cavalryman with grenades and bayonets, in addition to his ordinary gear, and to make of him practically a mounted infantryman. Trench warfare occupied most of the time and made nine-tenths of the discomforts of the soldiers of both armies. If proof of the adaptive capacity of the human animal were needed, it is afforded by the manner in which the men burrowed in vermin-infested earth and lived there under conditions of Arctic cold, frequently enduring long deprivations of food, fuel, and suitable clothing. During the early stages of the war, before men became accustomed to the rigors of the trenches, many thousands died as a direct result of the exposure. Many thousand of others were incapacitated for life by "trench feet," a group of maladies covering the consequences of exposure to cold and water which in those early days flowed in rivulets through most of the trenches. The trenches at Gallipoli had their own special brand of maladies. Heatstroke and a malarial infection were among these disabling agencies. Trench fever, a malady beginning with a headache and sometimes ending in partial paralysis and death, was another common factor in the mortality records. But in spite of all these and other discomforts, in spite of the disgusting vermin that crawled upon the men both in winter and in summer, both sides mastered the trenches and in the end learned to live in them with some degree of comfort.