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 character, intermediate in age.
But supposing for an instant, in this and other such cases, that the record of the first appearance 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.

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