But immense as was the growth of the artillery factor, the ultimate problem was the old problem of co-operation and combination of _all factors_. "Deep study of work other than one's own," "understanding of the other man's job"--for the highest success in any branch of the Army, these were and are indispensable. Only so can the vast machine work satisfactorily; only so can the human intelligence embodied in it come to its own. To the two subsidiary services most in the public eye--tanks and aeroplanes--I will return presently. As to the Signal Service, the "nervous system" of the Army, on which "co-operation and combination" depend, it has grown, says the Field Marshal, "almost out of recognition." At the outbreak of war it consisted of 2,400 officers and men; by the end of the war it had risen to 42,000. Cables, telegrams, wireless, carrier-pigeons and dog messengers--every kind of device was used for keeping up the communications, which mean everything in battle. The signal officer and his men creeping out over No Man's Land to mend a wire, or lay down a new one, in the very heart of the fighting, have carried the lives of thousands in their hands, and have risked their own without a thought. Sir Douglas Haig, from his Headquarters, spoke not only to every unit in the British Army, but to the Headquarters of our Allies--to London, Paris, and Marseilles. An Army Headquarters was prepared to deal with 10,000 telegrams and 5,000 letters in twenty-four hours; and wherever an army went, its cables and telephones went with it. As many as 6,500 miles of field cable have been issued in a single week, and the weekly average over the whole of 1918 was 3,000.