missing link
missing link {n.}
1. Something needed to complete a group; a missing part of a chain of things.
A 1936 penny was the missing link in John's collection of pennies.
The detective hunted for the fact that was the missing link in the case.
2. An unknown extinct animal that was supposed to be a connection between man and lower animals.
The missing link would be half man and half ape.
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Source: A Dictionary of American Idioms
