212 PHILOSOPHY OF ZOOLOGY. 
a. There isno determinate relation observed in the de- 
grees of perfection of the senses in different animals. Even 
in the same class, and subordinate divisions of a class, one 
species or genus may have the organs of smell very fully 
developed, another those of hearing, a third those of sight, 
while the other senses may be in a less perfect condition. 
Even among individuals of the same species, such differen- 
ces prevail. 
b. In judging of the properties of bodies, we seldom rest 
satisfied with the information obtained by one sense, but em- 
ploy the results furnished by the others, to correct or 
strengthen our conclusions. Thus, the organ of touch as- 
sists that of seeing, and sight aids the efforts of touch. 
Hence, as the senses mutually assist each other, it is dif- 
ficult to assign to each of them the knowledge which it 
has exclusively communicated. In the case of disease or 
accident, where one sense has been destroyed, the other 
senses, by an increased sensibility, in a great measure sup- 
ply the defect. Thus, hearing and touch, in many cases, 
supply the loss of sight, and exhibit striking displays of 
that compensating or repairing power, to which we have 
had frequent occasion to refer. 
c. In many animals, where some of the senses exist, al- 
though we are unable to detect the organs in which they 
are seated, as is the case with hearing in the annulose ani- 
mals, it is probable that the deficiency of one sense may 
be supplied by the sensibility of the remaining ones. 
d. The same qualities in bodies do not produce the same 
effects on the appropriate organs of all animals. There is 
a striking difference. in the degrees of sensibility ; so that 
an impression which would be overpowering to one animal 
is scarcely felt by another, as appears in the case of 
certain sounds, smells and lights. Even sensations excited 
by the same bodies in different animals, are dissimilar in 
