- Periods of gradual evolution and of efflorescence may be followed by stationary or senescent conditions.
- We have seen that different lines vary in vitality and in longevity, that from the earliest times senescent branches are given off, that different lines vary in the rate of evolution, that extinction is often heralded by symptoms of racial old age, which, however, vary widely in different groups.
This is not due to environmental conditions solely, because senescent branches of normal progressive groups are found in all geologic horizons, beginning, for gastropods, in the Lower Cambrian.
senescent cell is incapable of further propagation.
Once this cap reaches a certain length, cell division stops and the cell becomes senescent.