Final answer:
The purpose of the excerpt is to highlight humanity's dual nature and the balance of opposing traits within each person, framed within the cultural context of the narrative without labeling them strictly as good or evil.
Step-by-step explanation:
The main purpose of this excerpt from "The World on Turtle's Back" is to illustrate the dual nature of humanity, contrasting the concepts of the 'straight mind' and the 'crooked mind.' It explores the philosophical idea that these two opposing traits, akin to good and evil, are inherent in all people. The story uses the characters of the twins to symbolize these traits, explaining that both exist within every individual, without explicitly labeling them as 'good' or 'evil.' Instead, these aspects are presented as part of the human condition, influencing behaviors and shaping cultural perspectives.