<em><u>at the beginning of the film, the areas with fairly dense dots are mostly in places where the climate is good and the land is fertile. often along river banks and near deltas, people have developed forms of agriculture.</u></em>
Continental polar (cP) and maritime tropical (mT) air masses influence the weather of North America most.
The answer is seafloor spreading and subduction.
Hope this ain't too late (this is the PLATO answer)
