334 
GEOLOGICAL SUCCESSION. 
Chap. X, 
diagram are the modified offspring of those which lived 
at the fifth stage, and are the parents of those which 
became still more modified at the seventh stage; hence 
they could hardly fail to be nearly intermediate in 
character between the forms of life above and below. 
We must, however, allow for the entire extinction of 
some preceding forms, and in any one region for the 
immigration of new forms from other regions, and for a 
large amount of modification, during the long and blank 
intervals between the successive formations. Subject to 
these allowances, the fauna of each geological period 
undoubtedly is intermediate in character, between the 
preceding and succeeding faunas. I need give only one 
instance, namely, the manner in which the fossils of the 
Devonian system, when this system was first discovered, 
were at once recognised by palaeontologists as inter¬ 
mediate in character between those of the overlying 
carboniferous, and underlying Silurian system. But 
each fauna is not necessarily exactly intermediate, as 
unequal intervals of time have elapsed between con¬ 
secutive formations. 
It is no real objection to the truth of the statement, 
that the fauna of each period as a whole is nearly in¬ 
termediate in character between the preceding and 
succeeding faunas, that certain genera offer exceptions 
to the rule. For instance, mastodons and elephants, 
when arranged by Dr. Falconer in two series, first 
according to their mutual affinities and then according 
to their periods of existence, do not accord in arrange¬ 
ment. The species extreme in character are not the 
oldest, or the most recent; nor are those which are 
intermediate in character, intermediate in age. But 
supposing for an instant, in this and other such cases, 
that the record of the first appearance and disappearance 
of the species was perfect, we have no reason to believe 
that forms successively produced necessarily endure for 
