What is the theme in Endurance: Shackleton's Incredible Voyage by Alfred Lansing?

The dominant and obvious secondary theme of the text deals with the daily survival of men trapped by the reality of brutal nature. When Endurance sailed she was well-provisioned and while aboard the ship the members of the expedition lived a relatively easy life, free from undue worries of actual day-to-day survival. That changes, and drastically so, when Endurance is crushed by pack ice. Once the men abandon the crushed ship they are faced with the immediacy of simple day-to-day survival and all of their thoughts and actions are devoted to surviving until the next day.