This review is no exception and is more a quest for a single dominant mechanism that is able to account for many observations than it is a balanced exposition of the pros and cons of all mechanisms heretofore advanced but still not disproved.

• 这一回顾（评述）也不例外，与其说它是探求一个可以说明许多观测事实的主要机制，不如说它是对到目前为止提出的但仍未被否定的所有机制提出的正反两面意见的不偏不倚的介绍。