652 ANIMAL PHYSIOLOGY. 
ses of the vocal bands affect speech as well as voice, though 
to a less extent; and whispering is, of course, always possible. 
SPECIAL 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 circumstances of chance 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 there be no speech; 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 has 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., 
We may regard the development of a race of speaking ani- 
mals as dependent upon a corresponding advance in brain- 
structure, whether that was acquired by a sudden and pro- 
nounced variation, or by gradual additions of increase in cer- 
tain regions of the brain, or whether to the first there was then 
added the second. 
