The ancestor of the modern organ; pneumatic and hydraulic organs of classical antiquity--The organ in early mediæval times--The tenth and eleventh centuries: cloister and minster organs; the twelfth and thirteenth centuries: introduction of the ‘portative’ organ and balanced keys; the fourteenth century: chromatic keyboard; pedals; organ blowing--Fifteenth and sixteenth centuries; cathedral and church organs; the _Rückpositiv_; the Spanish _partida_; builders--The seventeenth century: mechanical development; tuning; union of manuals; the eighteenth century: the ‘Swell’; English builders; the Silbermanns--_Rococo_ adornment of cases; the nineteenth century and the birth of the modern instrument--Pneumatic action; electric action; the Universal Air Chest; duplex stop control; tonal improvements--The chamber organ; the concert organ; conclusion. Far back in the mist of ages some primal prototype of civilized man found that by blowing a hollow reed he produced a pleasing sound. This was probably the first step in the long process of evolution which has resulted in the concert organ of to-day. From the single reed of antediluvian times to the grouped reeds of the dawn of history was a logical transition; the early peoples of the Orient, the Egyptians, the Indians and the Chinese had accomplished it; but classical antiquity is, perhaps, our most definite point of contact, and it might be said that the bucolic Pan’s pipes or Syrinx of the Theocritan shepherd is the ancestor of the ‘king of instruments.’ The _Syrinx_ of pastoral Greece consisted of a series of reeds (tubes) without sound-holes, of graduated length and blown across the ends, each tube giving forth one note of the diatonic scale. In the course of time men hit upon the idea of allowing a bellows to take the place of the human lungs and thus produce sound by artificial instead of natural wind-pressure. Hence, even before the second century B. C. we have the first pneumatic organ--a series of variously tuned pipes, with mouthpieces, placed upon a box or chest, into which the air was pumped by bellows, the pipes sounding when the player opened the primitive valves which admitted the air to each pipe.