Some seeds only grow after they are pushed into the darkness of the earth. While exposed to the air and sun, the seed remains dormant. Once planted, the heat of the sun calls them out of the cold, and the seed feels the urgency to be born again. Similarly, in the previous eras, those seeds who could grow had done so. To grow the remaining seeds, they needed to be covered by darkness (the dark age). This is a karma, of course, because each seed (human seed) due to its previous actions had reduced itself to such a dense, senseless form that only extreme pressure (suffering) could cause it to realize its purpose. Yet, even now, most of the seeds remain senseless, and are being pulverized by the earth, and will fail to sprout.