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./