I have attributed the above sound to the gorilla, because I have been assured by many white men and scores of natives that it was made by him; but since my return from Africa I have had time to consider and digest certain facts tabulated on that trip, and as a result I am led to doubt whether this sound is made by the gorilla or not. There are good reasons to believe that it is made by the chimpanzee instead, and I shall state them. I observed that my own chimpanzees made this sound exactly the same as that I heard in the forest, except that it was less in volume, which was due to their age. I could induce them at any time to make the sound, and frequently did so in order to study it. On my arrival in New York I found that Chico, the big chimpanzee belonging to Mr. Bailey, frequently made the same sound at night. It was said to be so loud and piercing that it fairly shook the stately walls of Madison Square Garden. From reading the description given by the late Professor Romanes of the sound made by "Sally" in the London Gardens, it appears to be the same sound. It is well known to the natives that the chimpanzees beat on some sonorous body, which they call a drum. Four years ago I called attention to the habit of the two chimpanzees in the Cincinnati Gardens. They frequently indulged in beating upon the floor of their cage with their knuckles. This was done chiefly by the male. The late E. J. Glave described to me the same thing, as being done by the chimpanzees in the Middle Congo basin. It is not probable that two animals of different genera utter the same exact sound, and this is more especially true of a sound that is complex or prolonged. Neither is it likely that the two would have a common habit, such as beating on any sonorous body. Since it is certain that one of these apes does make the sound described, it is more than probable that the other does not. The same logic applies to the beating. Many things that are known of the chimpanzee are taken for granted in the gorilla, but it is erroneous to suppose that in such habits as these they would be identical. In some cases I have been able to prove quite conclusively that the chimpanzee alone did certain things which were ascribed to the gorilla. In view of these facts alone, I am inclined to believe that after all, the sound described is made by the chimpanzee and not by the gorilla.