314 
of the object itself. Compensatory 
reflexes of the eye muscles ensure that 
binocular fixation is unaffected by 
movements of the head. The resolv- 
ing power (acuity) of the human eye 
outside the fovea is poor, and the 
overwhelming importance of foveal 
vision to man is indicated by the fact 
that the area of the optical part of 
the brain concerned with vision by 
the fovea and its immediate neigh- 
borhood is many times larger than 
that devoted to the whole of the rest 
of the retina. It is stereoscopic foveal 
vision which gives solidity to the vis- 
ual field and makes accurate judg- 
ment of distance possible. On the 
other hand, man has retained some 
power of night vision, the legacy of 
his nocturnal ancestors, and rods are 
numerous in the periphery of the 
retina. 
Diurnal birds are much more fully 
diurnal than is man, and the extra- 
foveal cone population of their retinae 
is frequently denser than is that of 
the fovea of man, while rods are 
scarce. Consequently they are nearly 
blind at night and inactive unless 
disturbed. By day it may be inferred 
that their extra-foveal acuity is nearly 
as good as is man’s in the fovea. 
And they are thus relieved of the 
necessity of scanning the visual field 
bit by bit to build up a detailed picture, 
though intense scrutiny of objects of 
special interest with the central area 
of one eye or the other is achieved by 
movements of the whole head. And 
the detailed picture extends typically 
all around the horizon, for there is 
frequently an appreciable posterior 
as well as an anterior overlap of the 
monocular fields. ‘This all-around de- 
tailed picture cannot have the stereo- 
scopic solidity or the quality of dis- 
tance inherent in our own visual 
images. It is of some interest to en- 
quire how birds get along without 
this quality, and the answer seems to 
be that distance enters into avian 
vision with the same element of suc- 
cessiveness with which detail enters 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
into human vision. For man, the 
distance element enters into every 
glance, but the appreciation of detail 
of the field has to be built by a suc- 
cession of glances from the same point 
and directed to different parts of the 
field. For the bird, detail enters im- 
mediately into every glance, but the 
appreciation of distance has to be 
built up by a succession of glances 
from different points toward the same 
point of the field. 
This will, perhaps, seem a rather 
prolix way of saying that monocu- 
larity compels birds to employ the 
familiar method of parallax in esti- 
mating distance. But it does em- 
phasize the element of compromise 
inherent in sense organs, and the 
futility of any attempt to estimate 
which is the better of two types of 
sense organs on purely _ physical 
grounds. Man’s immediate visual 
awareness is three-dimensional but 
restricted, a bird’s is two-dimensional 
but virtually boundless. Which is the 
better is a meaningless question, for 
natural selection has ensured that 
each is the best possible in the circum- 
stances. If man had a bird’s eyes he 
would be incapable of threading a 
needle; if a bird had man’s he would 
be at the mercy of any cat which had 
the sense not to attempt a frontal 
attack. 
There is a good deal of evidence that 
both time and movement are necessary 
for the bird’s visual image to acquire a 
three-dimensional character. Small 
birds are frequently road casualties, 
but they are almost always overtaken 
on the ground. Once they have 
started to fly they are astonishingly apt 
at avoiding collision, and it is at least 
a plausible suggestion that they are so 
frequently run over because it is not 
till they have started to move that the 
nearness and speed of a vehicle are 
appreciated. Anyone who has watched 
a robin feeding will have noticed that 
it scrutinizes an insect monocularly 
from at least two places before peck- 
ing and eating it. During the peck the 
