34 THE DOCTRINE OF DESCENT. 
characteristics of form and internal structure; and it is 
very easy to select from every type forms in which the 
distinctive marks, comprised in the systematic diagnosis, 
may be displayed in full perfection. But this is imme- 
diately succeeded by a further observation, that of gra- 
dations within the type. When we previously compared 
the. polype and the bee, and were obliged to assign to 
each a very different rank, a portion of this difference 
of grade is certainly due to the difference of the family; 
but the forms united by family characteristics likewise 
diverge widely from each other, and the systematist 
speaks of lower and higher classes within every type, of 
lower and higher orders within every class. 
Reason is compelled to this by the same considerations 
which forced themselves upon us in the comparison of the 
polype and the bee. Why does the mussel stand lower 
than the snail? Because it does not possess a head, 
because its nervous system is not so concentrated and 
so voluminous, because its sensory organs are more de- 
fective. In one, as in the other, the structural material 
is present in quantities sufficient for the completion of 
the type ; but in the snail it is more developed, and the 
single circumstance of the integration of various parts to 
form the head confers a higher dignity upon the snail. 
It is needless to illustrate this gradation within the 
families by further examples ; the most superficial com- 
parison of a fish with a bird or a mammal, of one of 
the parasitic crustacea with a crayfish or an insect, 
shows, as the older zoology represented it, that in the 
actual forms the ground plan, or “ideal types,” find 
very diversified expression. 
A further result of this descriptive inquiry is the 
