Chap. XVIII. 
VOCAL OEGANS. 
277 
does not believe that the male is more noisy than the 
female. Hence, these latter monkeys probably nse their 
voices as a mutual call ; and this is certainly the case 
with some quadrupeds, for instance with the beaver.^ 
Another gibbon, the H. agilis^ is highly remarkable, 
from having the power of emitting a complete and 
correct octave of musical notes, ^ which we may reasonably 
suspect serves as a sexual charm ; but I shall have to 
recur to this subject in the next chapter. The vocal 
organs of the American Mycetes caraya are one-third 
larger in the male than in the female, and are wonder- 
fully powerful. These monkeys, when the weather is 
warm, make the forests resound during the morning and 
evening with their overwhelming voices. The males 
begin the dreadful concert, in which the females, with 
their less powerful voices, sometimes join, and which 
is often continued during many hours. An excellent 
observer, Eengger,^ could not perceive that they were 
excited to begin their concert by any special cause ; he 
thinks that like many birds, they delight in their own 
music, and try to excel each other. Whether most of the 
foregoing monkeys have acquired their powerful voices 
in order to beat their rivals and to charm the females — 
or whether the vocal organs have been strengthened 
and enlarged through the inherited effects of long-con- 
tinued use without any particular good being gained 
— I will not pretend to say ; but the former view, at 
least in the case of the Hylobates agilis, seems the most 
probable. 
I may here mention two very curious sexual pecu- 
liarities occurring in seals, because they have been sup- 
^ Mr. Green, in ‘ Journal of Linn. Soc.’ vol. x. Zoology, 1869, p. 362. 
® 0. L. Martin, ‘ General Introduction to the Nat. Hist, of Mamm. 
Animals,’ 1841, p. 431. 
' ‘ Naturgeschichte der Saugethiere von Paraguay,’ 1830, s. 15, 21. 
