Chap. XIII. 
CLASSIFICATION. 
431 
of any ancient and noble family, even by the aid of a 
genealogical tree, and almost impossible to do this 
without this aid, we can understand the extraordinary 
difficulty which naturalists have experienced in describ¬ 
ing, without the aid of a diagram, the various affinities 
which they perceive between the many living and ex¬ 
tinct members of the same great natural class. 
Extinction, as we have seen in the fourth chapter, 
has played an important part in defining and widening 
the intervals between the several groups in each class. 
We may thus account even for the distinctness of 
whole classes from each other—for instance, of birds 
from all other vertebrate animals—by the belief that 
many ancient forms of life have been utterly lost, 
through which the early progenitors of birds were 
formerly connected with the early progenitors of the 
other vertebrate classes. There has been less entire 
extinction of the forms of life which once connected 
fishes with batrachians. There has been still less in 
some other classes, as in that of the Crustacea, for 
here the most wonderfully diverse forms are still tied 
together by a long, but broken, chain of affinities. 
Extinction has only separated groups: it has by no 
means made them; for if every form which has ever 
lived on this earth were suddenly to reappear, though it 
would be quite impossible to give definitions by which 
each group could be distinguished from other groups, as 
all would blend together by steps as fine as those be¬ 
tween the finest existing varieties, nevertheless a natural 
classification, or at least a natural arrangement, would be 
possible. We shall see this by turning to the diagram : 
the letters, A to L, may represent eleven Silurian genera, 
some of which have produced large groups of modified 
descendants. Every intermediate link between these 
eleven genera and their primordial parent, and every 
