68 HUXLEY, ON CLASSIFICATION. 
laws when they rest on sufficiently repeated observations; so that 
now, whoso sees merely the print of a cleft foot may conclude that 
the animal which left this impression ruminated, and this conclusion 
is as certain as any other in physicsor morals. This footprint alone, 
then, yields to him who observes it, the form of the teeth, the form of 
the jaws, the form of the vertebre, the form of all the bones of the 
legs, of the thighs, of the shoulders, and of the pelvis of the animal 
which has passed by: it is a surer mark than all those of Zadig.” 
Morphological classification, then, acquires its highest importance 
as a statement of the empirical laws of the correlation of structures; 
and its value is in proportion to the precision and the comprehensive- 
ness with which those laws, the definitions of the groups adopted in 
the classification, are stated. So that, in attempting to arrive at 
clear notions concerning classification, the first point is to ascertain 
whether any, and if so, what groups of animals can be established, 
the members of which shall be at once united together and separated 
from those of all other groups, by well-defined structural characters. 
And it will be most convenient to commence the inquiry with groups 
of that order which are commonly called CuAssEs, and which are 
enumerated in an order and arrangement, the purpose of which will 
appear more fully by and by, in the following table. 
