GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 121 



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 inter- 

 mediate in character between the preceding and succeed- 

 ing faunas, that certain genera offer exceptions to the 

 rule. For instance, the species of mastodons and ele- 

 phants, 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 exist- 

 ence — do not accord in arrangement. The species ex- 

 treme in character are not the oldest or the most recent; 

 nor are those which are intermediate in character, inter- 

 mediate 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 else- 

 where subsequently produced, especially in the case of 

 terrestrial productions inhabiting 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 arrangement would not closely 

 accord with the order in time of their production, and 

 even less with the order of their disappearance; for the 

 parent rock-pigeon still lives; and many varieties between 



