His explanation was a combination of migration and descent with modification. He went on to say: "On this principle of inheritance with modification, we can understand how it is that sections of genera, whole genera, and even families are confined to the same areas, as is so commonly and notoriously the case.
What was the combination of effects that Darwin considered the most important in determining the differences in species that evolved separately in similar environments?
His explanation was a combination of migration and descent with modification.