432 
CLASSIFICATION. 
Chap. XIII. 
intermediate link in each branch and sub-branch of 
their descendants, may be supposed to be still alive; 
and the links to be as fine as those between the finest 
varieties. In this case it would be quite impossible to 
give any definition by which the several members of the 
several groups could be distinguished from their more 
immediate parents; or these parents from their ancient 
and unknown progenitor. Yet the natural arrangement 
in the diagram would still hold good ; and, on the prin¬ 
ciple of inheritance, all the forms descended from A, or 
from I, would have something in common. In a tree we 
can specify this or that branch, though at the actual 
fork the two unite and blend together. We could not, 
as I have said, define the several groups; but we could 
pick out types, or forms, representing most of the cha¬ 
racters of each group, whether large or small, and thus 
give a general idea of the value of the differences 
between them. This is what we should be driven to, if 
we were ever to succeed in collecting all the forms 
in any class which have lived throughout all time and 
space. We shall certainly never succeed in making 
so perfect a collection: nevertheless, in certain classes, 
we are tending in this direction; and Milne Edwards 
has lately insisted, in an able paper, on the high import¬ 
ance of looking to types, whether or not we can separate 
and define the groups to which such types belong. 
Finally, we have seen that natural selection, which 
results from the struggle for existence, and which almost 
inevitably induces extinction and divergence of character 
in the many descendants from one dominant parent- 
species, explains that great and universal feature in the 
affinities of all organic beings, namely, their subordina¬ 
tion in group under group. We use the element of 
descent in classing the individuals of both sexes and 
of all ages, although having few characters in common, 
