360 FOOT-NOTES TO EVOLUTION. 
trustworthy when it can not result in action. Objects 
too small to be touched are invisible to the eye. Objects 
beyond our reach, as the stars or the clouds, are not 
truthfully pictured. Accuracy of percep- 
Sensation truth- tion grows less as the square of the dis- 
ful in the degree tance increases. It is a recognised law 
that action is j : 
possible, of psychology that only medium varia- 
tions and differences are correctly esti- 
mated. The senses deal correctly only with the near, 
the mind only with the common. The unfamiliar lends 
itself readily to illusions. The familiar is recognised 
chiefly by breaks in continuity. The real forces of 
Nature are hidden by their grandeur, by their duration. 
Men see the waves on the surface of the sea, but not the 
mighty tides that move beneath it. Again, the senses 
are less acute than the mechanism of sense organs would 
make possible. This is shown through occasional cases 
of hyperesthesia or ultra-sensitiveness. This occurs in 
abnormal individuals or in diseased conditions. It oc- 
curs normally in creatures whose lives in some sense 
depend on it. Thus some of the most remarkable exhibi- 
tions of “mind reading” may be paralleled by retriever 
dogs, whose reason for existence is found in the hyper- 
zsthesia of the sense of smell. Hyperesthesia of more 
than one of the senses would be to most animals a 
source of confusion and danger rather than of safety. 
The high development of the brain in man in large de- 
gree takes the place of acuteness of special senses. It 
is part of the function of the will to regulate the senses 
and to suppress those impressions which should not lead 
to action. 
In his perception of external relations man is aided 
by the devices of science, which may be taken up or laid 
down at will. By means of instruments of precision any 
of the senses may be extended to an enormous degree, 
