CujLT.X. AFFINITIES OF EXTINCT SPECIES. 31 1 



facts with respect to the mutual affinities of the extinct forms 

 of life to each other and to livinf^ 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 of any 

 one great ])eriod in the earth's history will be intermediate 

 in g'eneral character between that which preceded and that 

 which succeeded it. Thus the species which lived at the sixth 

 p^reat stage of descent in the diagram are the modified ofTsjjring 

 of those which lived at the fifth stage, and are the parents of 

 1 hose which became still more modified at the seventh stage; 

 hence they could hardly fail to be nearly intermediate in char- 

 acter between the forms <jf life above and below. We must, hoAV- 

 ever, allow ior the entire extinction of some preceding forms, 

 and in any one region for the immigration of new fijrms from 

 other regions, and for a large amount of modification during 

 the long and blank intervals between the successive forma- 

 tions. Subject to these allowances, the fauna of each geolo- 

 gical 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 Devo- 

 nian system, when this system was first discovered, were at 

 once recognized by paleontologists as intermediate in charac- 

 ter between those of the overlying carboniferous, and under- 

 lying Silurian system. But each fauna is not necessarily ex- 

 actly 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, 

 mastodons and elephants, when arranged by Dr. Falconer in 

 two scries, first according to their mutual affinities and then 

 according to their periods of existence, do not accord in ar- 

 rangement. The species extreme in character are not the old- 

 est or the most recent ; nor are those which are intermediate 

 in character, intermediate in age. But supposing for an 

 instant, in this and other sucli cases, "that the recoril of the 

 lir^t appearance and disappearance of the species was perfect, 

 we have no reason to believe that forms successively produced 

 necessarily endure for corresponding lengths of time : a very 

 ancient form might occasionally last much longer than a form 

 elsewhere subsequently produced, csjiecially in the case of 



