306 
vertebrates, whereas it is difficult to 
recognize any similarity at all between 
the auditory parts of the ears of a fish 
and of a mammal. And though the 
eyes of vertebrates necessarily have the 
common structural plan to which they 
were committed at the dawn of their 
history, there is great variation in de- 
tail even in quite closely related ani- 
mals; and such attributes as color vision 
and the power of accommodation have 
been repeatedly lost and reacquired. 
It is not surprising that this should be 
so, for, given the ground plan of 
vertebrate organization, it is the per- 
formance of the distance receptors, 
more than any other single factor, 
which limits the field of an animal’s 
activities; and the evolution of new 
habits must go hand in hand with 
appropriate changes in the balance of 
that performance. 
The distance receptors of sight and 
hearing (we still know almost nothing 
of smell) are very far from being sim- 
ply physical devices whose function 
is ‘‘to tell the brain what is there.” 
Physical laws set an absolute limit 
to what they can do, but unless great 
care is taken in applying the laws in 
a biologically relevant way, it is found 
that their performance in some re- 
spects betters what a physicist would 
admit to be possible, while in others it 
fails mysteriously to achieve what he 
would consider easy and desirable. 
The reason is fairly evident to the biol- 
ogist. The organs of sight and hearing 
are vulnerable, bulky, and expensive 
to maintain, and natural selection 
ensures that their performance is 
pushed to the limit in those respects 
only which have survival value in the 
circumstances of the animal’s life; 
while refinements which are not so 
necessary, if they interfere with the 
primary requirements, or even if they 
only represent a significant overhead 
charge on the animal’s energy balance 
sheet, are dropped like a hot brick. 
Moreover the eye and, to a less 
extent, the ear are integrative centers. 
They do more than decode a pattern 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
of physical stimuli into terms of nerv- 
ous impulses. In some respects they 
stand in the same relation to the 
higher integrative centers in the brain 
as do subeditors to the editor of a 
newspaper. ‘Information received” 
is marshalled and the pursuit of fur- 
ther information is organized without 
reference to the editor’s office, which 
gets only carefully selected and docu- 
mented items. The greater part of the 
incoming news is filed or blue-penciled 
before it has a chance of affecting the 
policy of the paper as a whole. The 
distance receptors are, therefore, far 
from being the “‘passive linear systems” 
of a physicist’s dream. 
It is easier to interpret the structure 
and function of the distance receptors 
and to understand some of their limi- 
tations if they are looked at historically 
for, though they are rebuilt in each 
generation, they are not redesigned. 
The basic specification, hundreds of 
millions of years old, is smothered 
with emendations, additions, and dele- 
tions, but it is still there; and all the 
changes have represented workable 
modifications within the framework 
of the original design. The unborn 
baby (and the unhatched bird) has the 
task of fashioning for each ear a fre- 
quency analyzer of great sensitivity, 
resolving power, and range from a 
design, inherited from its fishlike an- 
cestors, which is basically the design 
for a seismograph. Trivial modifica- 
tions may have occurred in any or 
every generation, but there must al- 
ways have been continuity, and never 
an opportunity to scrap and _ start 
again. The finished job is naturally 
very different from a physicist’s con- 
ception of what it ought to be. But 
for the biologist it is a delight to find 
what perfection is achieved with the 
means available and how subtly the 
most unlikely means are used—how, 
for example, the ear of the mammal 
has taken over a few superfluous 
bones and muscles from the reptilian 
jaw and used them to make what an 
engineer would recognize as an im- 
