The speaker in Shakespeares marriage sonnet 11 titled As fast as thou shalt wane so fast thou growst is again reminding the young swain that he will eventually grow old and whither, but he tries to mitigate that sad state of affairs for the young man by telling him that even as withers, if he will produce offspring the young man will continue vibrantly in his heir. And the speaker tells the young man that he may rightly think of his blood as the fresh blood in his offspring, even as the young man himself changes from a young man to an old man.
In producing a child, the young man will be acting wisely; his act will be beautiful, and he will become productive. While the opposite, that is, growing old and failing to leave an heir will result in folly, because in old age the young man will merely surrender to cold decay. Then the speaker asserts that if everyone thought this way, taking this selfish path and remaining single and childless, the end of time would occur and in only sixty years or two generations there would be no world in existence.
Then the speaker muses that those who are not worthy, those who are harsh, featureless, and rudewell, let them barrenly perish. If one is not a pleasing individual with worthwhile qualities, then it is fitting that such a one should not marry and father children, but he asks the young man to realize that the young man is exceptional, he is gifted by nature bounteously, and he should cherish that bounty, not let it slip away without passing on those pleasing qualities.
The couplet employs a printing metaphor: referring to nature, he says, She carvd thee for her seal, and meant thereby / Thou shouldst print more, not let that copy die. Nature Click here for more. has made you her seal, and the young man, according to the speaker, is obligated to print more, not let that copy die. Again, he is urging the young man to understand his obligation to the world, to nature, and to himself. He must perpetuate his bounteous gifts by fathering a child who will also possess those gifts.