SO ENDS this two weekend section on imperfection in nature. One striking lesson is that the development of some bad traits are irreversible, such as molting in insects, and this can forever block a future work-around that could lead to a lineage without molting. Ninety nine percent of all species that ever existed went extinct; for sure some were by catastrophic events, but many must have been by a fatal irreversible trait that locked them into an evolutionary dead end.