III ON ANIMAL LIFE 87 
We are too apt to suppose that the senses 
of animals must closely resemble, and be con- 
fined to ours. 
No one can doubt that the sensations of 
other animals differ in many ways from ours. 
Their organs are sometimes constructed on 
different principles, and situated in very un- 
expected places. There are animals which 
have eyes on their backs, ears in their legs, 
and sing through their sides. 
We all know that the senses of animals are 
in many cases much more acute than ours, as 
for instance the power of scent in the dog, of 
sight in the eagle. Moreover, our eye is 
much more sensitive to some colours than to 
others ; least so to crimson, then successively 
to red, orange, yellow, blue, and green; the 
sensitiveness for green being as much as 750 
times as great as for red. This alone may 
make objects appear of very different colours 
to different animals. 
Nor is the difference one of degree merely. 
The rainbow, as we see it, consists of seven 
colours — red, orange, yellow, green, blue, 
indigo, and violet. But though the red and 
