AFFINITIES OF EXTINC2 SPECIES. 361 



existing members of the same groups; and tliis by tlio 

 concurrent evidence of our best palseontologists is fre- 

 quently the case. 



Thus, on the theory of descent witb modification, tlio 

 main facts with respect to the mutual aOiiiiiies of tbo 

 extinct forms of life to each other and to living forms, 

 are explained in a satisfactory manner. And they are 

 wholly inexplicable on any other view. 



On this same theory, it is evident that the fauna during any 

 one great period in tiie earth's history will be intermediate 

 in general character between that which preceded 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 fiftli stage, 

 and are the parents of those which became still more 

 modihed at the seventh stao^e; hence thev could hardlv fail 

 to be nearly intermediate in character between the forma 

 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 otlier 

 regions, and for a large amount of modification during the 

 long and blank intervals between the successive formationd. 

 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 svstem, when this svstem was first discovered, 

 were at once recognized by palaeontologists as intermodiato 

 in character between those of the overlying carboniferous 

 and underlying Silurian systems. But each fauna is not 

 necessarily exactly intermedi ite, as unequal intervale of 

 time have elapsed between consecutive formations. 



It is no real objection to the truth of the statement that 

 the fauna of each period as a whole is nearly intennediato 

 in character between the preceding and succeeding fauna?, 

 that certain genera offer exceptions to tbe 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 existence— do not accord in arrangement. 

 The species extreme in character are not the oldest or the 

 moat recent; nor are those which are intermediate m ciiar- 



