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 my first voyage, and, as a result of these reflections, I doubt whether this sound is made by the gorilla. There are reasons to believe that it is made by the chimpanzee. I observed that my own chimpanzees made a sound exactly the same as that I heard in the forest, except that it was less in volume. This was due to the age of the apes that made it. I could induce them at any time to make the sound, and frequently did so in order to study it. After my arrival in New York I found that Chico--the big chimpanzee belonging to Mr. Bailey--frequently made the same sound. This he always did at night. The cry 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 that she made the same sound. It is well known to the natives that chimpanzees beat on some sonorous body, which the natives call a drum. In 1890 I called attention to the beating practiced by the two chimpanzees in the Cincinnati Gardens. They frequently indulged in beating with their knuckles upon the floor of their cage. 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 to 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 view of the facts I am inclined to believe the sounds described are made by the chimpanzee and not by the gorilla.