AFFINITIES OF EXTINCT SPECIES. 339 



that which succeeded it. Thus the species which lived at 

 the sixth great stage of descent in the diagram are the modi- 

 lied 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 extinc- 

 tion 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 un- 

 doubtedly is intermediate in character, between the preced- 

 ing 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 palaeontologists 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 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 

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

 acter, intermediate in age. But supposing for an instant, in 

 this and other such cases, that the record of the first appear- 

 ance and disappearance of the species was complete, which 

 is far from the case, we have no reason to believe that forms 

 successively produced necessarily endure for corresponding 

 lengths of time. A very ancient form may occasionally have 

 lasted much longer than a form elsewhere subsequently pro- 

 duced, especially in the case of terrestrial productions inhab- 

 iting separated districts. To compare small things with 

 great; if the principal living and extinct races of the 

 domestic pigeon were arranged in serial affinity, this arrange- 

 ment would not closely accord with the order in time of 

 their production, and even less with the order of their dis« 



