What is the theme in Lucifer's Hammer by Larry Niven?

Self-preservation is another theme in the story. While many of the characters exhibit a strong desire to live, some show that they will persevere no matter what life throws at them. One of the best examples of this is seen in Hugo Beck. Hugo is expelled from the stronghold and spends some time among those on the outside. He's eventually becomes part of the New Brotherhood Army, where he's told that he will eat of human flesh or be killed. Several refuse to participate in the cannibalistic ritual, but Beck and others do so.