Chap. X. 
AFFINITIES OF EXTINCT SPECIES. 
331 
JBatrachians, the older Fish, the older Cephalopods, and 
the eocene Mammals, with the more recent members 
of the same classes, we must admit that there is some 
truth in the remark. 
Let us see how far these several facts and inferences 
accord with the theory of descent with modification. As 
the subject is somewhat complex, I must request the 
reader to turn to the diagram in the fourth chapter. 
We may suppose that the numbered letters represent 
genera, and the dotted lines diverging from them 
the species in each genus. The diagram is much too 
simple, too few genera and too few species being 
given, but this is unimportant for us. The horizontal 
lines may represent successive geological formations, 
and all the forms beneath the uppermost line may 
be considered as extinct. The three existing genera, 
will form a small family; and a 
closely allied family or sub-family; and a 
third family. These three families, together with the 
many extinct genera on the several lines of descent 
diverging from the parent-form (A), will form an order; 
for all will have inherited something in common 
from their ancient and common progenitor. On the 
principle of the continued tendency to divergence of 
character, which was formerly illustrated by this dia¬ 
gram, the more recent any form is, the more it will 
generally differ from its ancient progenitor. Hence we 
can understand the rule that the most ancient fossils 
differ most from existing forms. We must not, how¬ 
ever, assume that divergence of character is a necessary 
contingency; it depends solely on the descendants from 
a species being thus enabled to seize on many and dif¬ 
ferent places in the economy of nature. Therefore it is 
quite possible, as we have seen in the case of some 
Silurian forms, that a species might go on being slightly 
modified in relation to its slightly altered conditions of 
