What are the motifs in The Day of the Triffids by John Wyndham?

A recurring idea is the frailty of human existence. This is explored through action, character, relationship, dialogue, and the triggering of the reader's thought process. It might, in fact, be considered a key component of that primary theme, in that the choice between physical and moral survival in the novel is essentially defined by frailty - the protection of physical frailty, the fear of moral frailty, and the realization of emotional frailty.