332 
GEOLOGICAL SUCCESSION. 
Chap. X. 
life, and yet retain throughout a vast period the same 
general characteristics. This is represented in the dia¬ 
gram by the letter F 
All the many forms, extinct and recent, descended 
from (A), make, as before remarked, one order; and this 
order, from the continued effects of extinction and 
divergence of character, has become divided into se¬ 
veral sub-families and families, some of which are 
supposed to have perished at different periods, and some 
to have endured to the present day. 
By looking at the diagram we can see that if many 
of the extinct forms, supposed to be embedded in the 
successive formations, were discovered at several points 
low down in the series, the three existing families on the 
uppermost line would be rendered less distinct from each 
other. If, for instance, the genera m®, m^, 
were disinterred, these three families would be so closely 
linked together that they probably would have to be 
united into one great family, in nearly the same manner 
as has occurred with ruminants and pachyderms. Yet 
he who objected to call the extinct genera, which thus 
linked the living genera of three families together, inter¬ 
mediate in character, would be justified, as they are 
intermediate, not directly, but only by a long and cir¬ 
cuitous course through many widely different forms. If 
many extinct forms were to be discovered above one 
of the middle horizontal lines or geological formations 
—^for instance, above No. YI.—but none from beneath 
this line, then only the two families on the left hand 
(namely, &c., and 5^^ &c.) would have to be united 
into one family; and the two other families (namely, 
to now including five genera, and to would 
yet remain distinct. These two families, however, would 
be less distinct from each other than they were before the 
discovery of the fossils. If, for instance, we suppose the 
existing genera of the two families to differ from each 
