missing link
noun
1- a thing that is needed in order to complete a series, provide continuity, or gain complete knowledge（系列关联事物中）缺少的一环：
she is the missing link between the European ballad tradition and Anglo-American white soul.
在欧洲传统民谣和英美白人爵士灵乐之间她起了承上启下的作用。
1.1
- a hypothetical fossil form intermediate between two living forms, especially between humans and apes缺环（指介于两种生命形式, 尤指人类和类人猿之间的假想化石形态）。