334 GEOLOGICAL SUCCESSION. Chap. X. 



mediate in general character between that which pre- 

 ceded and that which succeeded it. Thus, the species 

 which lived at the sixth great stage of descent in the 

 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 for the coming in of quite 

 new forms by immigration, and for a large amount of 

 modification, during the long and blank intervals be- 

 tween the successive formations. Subject to these allow- 

 ances, 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 intermediate in 

 character between those of the overlying carboni- 

 ferous, 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, tliat 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 



