When the chimpanzee attacks, so far as I have seen among my own specimens, he approaches his enemy and strikes with both hands, one slightly in advance of the other. After striking a few blows, he will grasp his opponent and use his teeth, then shoving him away again uses his hands, and usually, on beginning the attack, accompanies the assault with a loud, piercing scream. Neither he nor the gorilla closes the hand to strike, nor uses any weapon except the hands and teeth. I had another young female gorilla for a short time as a subject for study. Her mode of attack appeared to be the same, but she was too large to risk in such experiments. I have read and heard descriptions of the sounds made by the gorilla, but nothing ever conveyed to my mind an adequate idea of their true nature, until I heard them myself within a few hundred feet of my cage in the dead of night. By some it has been called roaring, and by others howling; but it is neither truly a roar nor a howl. They utter a peculiar combination of sounds, beginning in a low, smooth tone, which rapidly increases in pitch and frequency, until it becomes a terrific scream. The first part of the series is quite within the scope of the human voice, but as it rises in pitch and increases in volume it passes far beyond the reach of the human lungs. The first sound of the series and each alternate sound is made by expiration, while the intermediate ones appear to be by inspiration, but how it is accomplished is difficult to say. The sound as a whole resembles the braying of an ass, except the notes are shorter, the climax higher, and the sound is louder. A gorilla does not yell in this manner every night, but when he does so it is usually between two and five o'clock in the morning; I have never heard the sound during the day nor in the early part of the night. When he thus screams, he repeats the series from ten to twenty times, at intervals of one or two minutes each. I know of nothing in the way of vocal sounds that can inspire such terror as the voice of the gorilla. It can be heard over a distance of three or four miles. I could assign no definite meaning to it unless it was intended to alarm some intruder that came too near. One morning between three and four o'clock I heard two of them screaming at the same time. I do not mean to say at the same instant, but at intervals during the same period of time. One of them was within about a third of a mile of me, and the other in another direction perhaps a mile away. The points we occupied respectively formed a scalene triangle. The sounds did not appear to have any reference to each other. Sometimes they would alternate, and at other times they would interrupt each other. They were both made by giants of their kind, and every leaf in the forest vibrated with the sound. This was during the latter part of May. They do scream in this way from time to time throughout the year, but it is most frequent and violent during February and March.