The phrase "intermediate form" refers to something that is in between two other things or states. It is a stage or condition that comes after the initial state but before the final state. Full definition
This brings up the problem as to the survival probabilities of intermediate forms before they are fully functional. (faith.org.uk)
He nonetheless firmly believed that the eye did evolve in that way, despite a lack of evidence for intermediate forms at the time. (scientificamerican.com)
Plenty of fossils of intermediate forms have been found. (firstthings.com)