What are the motifs in Earth Abides by George R. Stewart?

Adaptation is a constant motif in dystopian fiction. Ish was a young man when he was confronted with the reality that everyone he knew was gone. He seems to quickly accept the fact that the greater portion of the population is gone and sets himself up to observe what will happen next. As he realizes that there are no doctors and no hospitals left, he is suddenly overwhelmed by the idea that even a simple injury could result in death.