600 ANIMAL PHYSIOLOGY. 
organ known, is of peculiar shape as a whole, presenting a large 
posterior surface for retinal expansion; a very convex cornea, 
a highly developed lens, an extremely movable iris; eyelids 
and a nictitating membrane (third eyelid), which may be made 
to cover the whole of the exposed part of the eye, and thus 
shield screen-like from excess of light; ossifications of the scle- 
rotic; a structure which is a peculiar 
modification of the choroid, of which it 
is a sort of offshoot and like it very 
vascular, answering to the falciform 
process of the eye of the fish and the 
reptile. From its appearance it is 
termed the pecten. Birds, on account 
of a highly developed ciliary muscle, 
possess wonderful powers of accommo- 
dation, rendered important on account 
of their rapid mode of progression, 
They also seem to be able to alter the 
Fic. 443.—Eye of nocturnal bird size of the pupil at will. 
Cor eraea © phe tye Evilution. ‘From the above brief ac- 
ina; P. pecten; No, optic a : 
nerve ; Sc, ossification of scle. count of the eye in different grades of 
rotic coat; CM, ciliary mus- a . = . . 
cle. Birds have unusually animals, it will appear that its modifi- 
keen vision, great power of : . : 
accommodation, andextreme cations answer to differences in the 
mobility of the iris. : 
environment. 
Adaptation is evident. Darwin believes this to have been 
effected partly by natural selection—i. e., the survival of the 
animal in which the form of eye appeared best adapted to its 
needs, and partly by the use or disuse of certain parts. 
The latter is illustrated—1. By the blind fishes, insects, etc., 
of certain caves, as those of Kentucky; and it is of extreme in- 
terest to note that various grades of transition toward complete 
blindness are observable, according to the degree of darkness in 
which the animal is found living, whether wholly within the 
cave or where there is still some light. A crab has been found 
with the eye-stalk still present, but the eye itself atrophied. 
Again, animals that burrow seem to be in process of losing 
their eyes, through inflammation from obvious causes ; and some 
of them, as the moles, have the eye still existing, though well- 
nigh or wholly covered with skin. Internal parasites are often 
without eyes. It is not difficult to understand how one bird of 
prey, with eyes superior to those of its fellows, would gain 
supremacy, and, in periods of scarcity, survive and leave off- 
spring when others would perish. 
