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 grasps his opponent and uses his teeth. Then, shoving him away, he again uses the hands. Usually, on beginning the attack, he accompanies the assault with a loud, piercing scream. Neither he nor the gorilla closes the hand to strike or uses any weapon except the hands and the teeth. I have read and heard descriptions of the sounds made by gorillas, but nothing ever conveyed to my mind an adequate idea of their real nature until I heard them myself within about a 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 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 sound of the series and each alternate sound are made by expiration; the intermediate ones appear to be by inspiration. How this is accomplished it is difficult to say. The sound as a whole resembles the braying of an ass, except that the notes are shorter, the climax is 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 screaming he repeats the series from ten to twenty times, at intervals of one or two minutes apart. 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 can assign no definite meaning to it unless it is intended to alarm some intruder. One morning, between three and four o’clock, I heard two of them screaming at the same time. I do not mean 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 respectively occupied formed a scalene triangle. The sounds made by the two apes 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 scream in this way from time to time throughout the year, but it is most frequent and violent during February and March.