618 : ANIMAL PHYSIOLOGY. 
utricle containing many otoliths, and a small dilatation of the 
vestibule, which may indicate an undeveloped cochlea. The 
Fie. 461.—Otoliths from various animals (after Riidinger). 1, from goat; 2, herring; 3, 
devil-fish ; 4, mackerel ; 5, flying-fish ; 6, pike; 7, carp; 8, ray; 9, shark ; 10, grouse. 
membranous labyrinth is contained in a periotic capsule, partly 
bony and partly cartilaginous, which is supplied with per- 
ilymph. There is a fenestra ovalis, but not a fenestra rotunda, 
though the latter is present in reptiles. In crocodiles and 
birds the cochlea is tubular, straight, and divided into a scala 
tympani and a scala vestibuli. The columella of lower forms 
still persists. In birds and mammals the bone back of the ear 
is hollowed out to some extent and communicates with the 
tympanum. Except among the very lowest mammals (Echid- 
na), the ear is such as has been described in detail already. 
Evolution.—The above brief description of the auditory organ 
in different groups of the animal kingdom will suffice to show 
that there has been a progressive development or increasing 
differentiation of structure, while the facts of physiology point 
to a corresponding progress in function—in other words, there 
has been an evolution. No doubt natural selection has played 
a great part. -It has been suggested that this is illustrated by 
cats, that can hear the high tones produced by mice, which 
would be inaudible to most mammals; and, as the very exist- 
ence of such animals must depend on their detecting their prey, 
it is possible to understand how this principle has operated to 
determine even what cats shall survive. The author has noticed 
