Turning for a very brief space to Van Winkle, and why we can clearly understand why some species, once lost, should never reappear

In members of the same class the average amount of change during long and equal periods of time, may, perhaps, be the same, but as the accumulation of the long-enduring formations depends upon great masses having been deposited on areas while subsiding, our formations have been almost necessarily accumulated at wide and irregularly intermittent intervals; consequently, the amount of change exhibited (embedded in consecutive formations) is not equal; each formation does not mark a new and complete act of creations but only an occasional scene, taken almost at hazard, in a slowly changing drama ...