MAMMALS 63 



was attributed to the same cause, but this is a misconception. 

 The attack of the jaws directed to the back of the neck would 

 result in the seizing of the hair in most cases by the teeth, 

 and this pulling of the hair would stimulate the follicles, 

 while on the cheeks the hair would be too short to be grasped, 

 and the skin tightened over the bones would be scratched 

 and torn. It is also possible that the hairs of the mane are 

 erected by the skin muscles in male baboons, as they certainly 

 are in the lion, and this would add to the irritation of the 

 hair follicles, but it does not seem likely that this action is 

 the sole cause of the development of the mane. 



The voice is developed to an extraordinary degree in the 

 South American monkeys of the genus Mycetes, and the vocal 

 organs have a corresponding development. Both voice and 

 larynx are considerably more developed in the males than in 

 the females, but in accordance with the general rule when a 

 character is present in both sexes, the peculiarities are 

 developed not at maturity, but in the embryo before birth. 

 The case, however, deserves mentioning, because it indicates 

 in so convincing a manner the connection of the modification 

 with the mechanical effects of the action to which it is sub- 

 servient. The anatomy and evolution of the vocal organs in 

 these howling monkeys have been discussed by the dis- 

 tinguished anatomist Dr. Hans Gadow, 1 on whose authority 

 the following facts are stated. The thyroid cartilage and 

 basihyal cartilage are hollowed out and expanded into large 

 globular receptacles which are occupied by air sacs opening 

 from the larynx. These cavities act as resonators, and to 

 them the extraordinary volume of sound produced is due. 

 It is difficult, if not impossible, to conceive how such structures 

 could have been evolved, except on the view that the form of 



1 "Description of the Modifications of certain Organs which seem to be 

 Illustrations of the Inheritance of Acquired Characters," Zoologische Jahr- 

 biicher, Bd. V. 



