268 SYSTEM OF AGASSIZ. 
The system upon which M. Agassiz has estab- 
lished his classification of recent Fishes is in a 
peculiar degree applicable to fossil Fishes, being 
founded on the character of the external cover- 
ings, or Scales. This character is so sure and 
constant, that the preservation, even of a single 
scale, will often announce the genus and even 
the species of the animal from which it was 
derived; just as certain feathers announce to a 
skilful ornithologist the genus or species of a 
Bird. It follows still further, that as the nature 
of their outward covering indicates the relations 
of all animals to the external world, we derive 
from their scales certain indications of the rela- 
tions of Fishes ;* the scales forming a kind of 
external skeleton, analogous to the crustaceous or 
* The foundation of this character is laid upon the dermal 
covering, the skin being that organ which, more than any other 
part of the body, shows the relation of every animal to the ele- 
ment in which it moves. 
The form and conditions of the feathers and down show the 
relation of Birds to the air in which they fly, or the water in which 
they swim or dive. The varied forms of fur and hair and bristles 
on the skins of Beasts are adapted to their respective place, and 
climate, and occupations upon the land. The scales of Fishes 
show a similar adaptation to their varied place and occupations 
beneath the waters. 
Mr. Burchell informs me that he has observed, both in Africa 
and South America, that in the order of Serpents a peculiar 
character of the scales appears to indicate a natural subdivision ; 
and that in that tribe, to which the Viper, and nearly all the 
venomous Snakes belong, an acute ridge, or carina^ along each 
dorsal scale may be considered as a distinctive mark. 
