missing link

[missing link] n. 1. Something needed to complete a group; amissing part of a chain of things.

A 1936 penny was the missinglink in John's collection of pennies.

The detective hunted for thefact that was the missing link in the case.

2. An unknown extinctanimal that was supposed to be a connection between man and loweranimals.

The missing link would be half man and half ape.