missing link
noun [ S ] uk/ˌmɪs.ɪŋ ˈlɪŋk/ us/ˌmɪs.ɪŋ ˈlɪŋk/
（一系列相关联事物中）必要的环节
something that is necessary to complete a series or solve a problem
Those documents provided the missing link, and the police were able to make an arrest soon after they discovered them. 这些文件提供了必要的证据，警察发现之后很快就实施了逮捕。
the missing link 缺环（据推定存在于类人猿和人类之间的过渡动物）
an animal that no longer exists or might never have existed and is thought to explain how humans developed from apes