We have a time of a mingling of diverse populations. We also have a time of gestation. The term gestation is appropriate. Gestation generally takes place in darkness and it takes place without being known. A seed gestates underground.
Fertilization is a process which takes place gradually and in secret.
What occurs around the 3rd and 4th century until about the 7th and 8th century is this intermingling of the Roman or ancient classical cultures and the various barbarian peoples and institutions. Law, for example became a blending of Roman law and the law of the Germans, particularly as it affected the administration of the manor. Church law remained very close to the Roman law. But, the law of the land tended to be a mingling of ancient Roman and of Germanic ideas. As far as blood lines were concerned intermarriage became common.