Read it through once

From all this it is clear that in order to carry plants to new countries and new homes, everything that moves on the earth's surface can be employed. Not only the wind, but ocean currents, river waters, icebergs, and floating ice are used. Migrating birds, mammals, and especially the most restless and unsettled animal of all, viz. man, are at work consciously and deliberately, or unconsciously and accidentally, carrying the seeds to form new forest, grasslands, or harvests in other countries.