AFFINITIES OF EXTINOT SPECIES. 361 



existing members of the same groups; and this by the 

 concurrent evidence of our best palaeontologists is fre- 

 quently the case. 



Thus, on the theory of descent with modification, the 

 main facts with respect to the mutual aftinities of the 

 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, itis evident that the fauna during any 

 one great period in the 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 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 recognized by paleontologists as intermediate 

 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 consecutive 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 accordiiig 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 

 most recent; nor are those which are intermediate in char- 



