1.   A major difficulty arises when fossil species disappear for good as the physical environment, over millions of years, inevitably changes.

2.   Difficulties have arisen as to whether letters written to Ministers by M.P.s in the course of their duties are covered by absolute privilege.

3.   Difficulties sometimes arise when the agency redefines pollution control policy for a stretch of water and, wishes to tighten existing standards.

4.   In some cases, difficulties arise because of a combination of less individualisation and low survival.

5.   In this section, I discuss other traits for which this difficulty arises.