Re-re-explanation:
If the two species had a distant relative, but diverged over time as a result of differing stresses that the environment places on them, then their adaptive features would evolve differently and separately to suit their environment.
But if the environment gradually changed (because of a changing environment over time or from migration) so that they placed similar stresses on the differing species, the species would slowly evolve to become much more similar in their adaptive features. Thus, they are converging once again into a common form.
I think I've got it that time.