600 COMPARATIVE PHYSIOLOGY. 



pitch, quality, and intensity, some species croaking so as to be 

 heard at the distance of at least a mile. It is a matter of easy 

 observation that when frogs croak the capacity of the mouth 

 cavity is greatly increased, owing to the distention of resonat- 

 ing sacs situated at each angle of the jaws. When tree-frogs 

 croak, their throats are greatly distended, apparently in suc- 

 cessive waves. 



SFECIAI. CONSIDERATIONS AND SUMMARY. 



Evolution.— The very lowest forms, and in fact most inverte- 

 brate groups, seem to be voiceless. Darwin has shown that 

 voice is, in a large number of groups, confined either entirely 

 to the male, or that it is so much more developed in him as to 

 become what he terms a " sexual character. " There is abundant 

 evidence that males are chosen as mates by the females, among 

 birds especially, not alone for superiority in beauty of plumage, 

 but also for their song. Thus, by a process of natural selection 

 (sexual selection), the voice would tend to improve with the 

 lapse of time, if we admit heredity, which is an undeniable fact, 

 even among men — whole families for generations, as the Bachs, 

 having been musicians. 



One can also understand why on these principles voice 

 should be especially developed in certain groups (birds), while 

 among others (mammals) form and strength should determine 

 sexual selection, the strongest winning in the contests for the 

 possession of the females, and so propagating their species under 

 the more favorable circumstance of choice of the most desira- 

 ble females. 



Pathology teaches that, when certain parts of the brain 

 (speech-centers) of man are injured by accident or disease, the 

 power of speech may be lost. From this it is evident that the 

 vocal apparatus may be perfect and yet speech be wanting; so 

 that it becomes comprehensible that the vocal powers of, e. g., 

 a dog, are so limited, notwithstanding his comparatively highly 

 developed larynx. He lacks the energizing and directive ma- 

 chinery situated in the brain. 



Some believe that there was a period when man did not pos- 

 sess the power of speech at all ; and many are convinced that 

 the human race have undergone a gradual development in this 

 as in other respects. Certain it is that races differ still very 

 widely in capacity to express ideas by spoken words. 



