In 'If I Forget Thee, O Earth', what, according to Marvin's father, is necessary for the Colony to survive?
Marvin’s father explains to him properly for the first time about the devastation that overtook earth, leaving their little colony on the moon as the only survivors. It is far too dangerous, as yet, to even think of returning to earth, which Marvin sees lit up by an ‘evil phosphorescence’; like much other science-fiction of the mid-20th century the story taps into prevailing fears about a nuclear holocaust.
He was looking upon the funeral pyre of a world - upon the radioactive aftermath of Armageddon.
The colonists are therefore stranded on the moon, but Marvin now realises that they do plan to return – eventually.
He would never walk beside the rivers of that lost and legendary world, or listen to the thunder raging above its softly rounded hills. Yet one day – how far ahead? – his children’s children would return to claim their heritage.
Until then, the colony does have the physical means to survive on the moon. However, Marvin’s father stresses that that this is not the most important thing:
But unless there was a goal, a future towards which it could work, the Colony would lose the will to live, and neither machines nor skills nor science could save it then.
The will to survive then, is what essentially matters. It is faith and hope which really sustain life, not physical and technological resources. The Colony has to keep the hope alive and not succumb to despair. The return to earth is the goal, no matter how far in the future it may be. Marvin realises that he too has to keep the faith, and pass it on to his descendants.
In its mourning for a lost world, the story is deeply elegiac, but at the same time it does sound a note of optimism for the future.