Chap. XL] AFFINITIES OP EXTINCT SPECIES. 113 



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 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 systems. But 

 each fauna is not recessarily 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 intermediate 

 in character between the preceding and succeeding 

 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 theu' periods of existence, 

 — 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 character, inter- 



