Chap. XI.J APPINWIES OP EXTINCT StEClBg. 113 



cies 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 forme of life above and below. 

 We must, however, allow for the entire extinction of some 

 preceding forms, and in any one region for the immi- 

 gration 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 systems. But 

 each fauna is not necessarily exactly intermediate, as 

 unequal intervals of time have elapsed between conse- 

 cutive formations. 



It is no real objection to the truth of the statement 

 that the fauna of each period as a whole is nearly inter- 

 mediate in character between the preceding and suc- 

 ceeding faunas, that certain genera offer exceptions to 

 the rule. For instance, the species of mastodons and 

 elephants, when arranged by Dr. Falconer in two series, 

 — ^in the first place according to their mutual affinities, 

 and in the second place according to their periods of ex- 

 istence, — do not accord in arrangement. The species 

 extreme in character are not the oldest or the most 

 recent; nor are those which are intermediate in charac- 



