Final answer:
Spender's admission of not feeling all Martian signifies his internal conflict with identity and the yearning for his own culture, representing a core theme of human nature and introspection.
Step-by-step explanation:
When Spender admits he doesn't feel all Martian after all, he's admitting a sense of disconnection from the alien culture he's been trying to understand and assimilate with. This sentiment reflects a universal human experience—the struggle between the desire to embrace new experiences and the pull of one's own identity and roots. The excerpt suggests a theme of human nature and existential reflection, with characters contemplating their personal identity in the wake of overwhelming circumstances such as being isolated in a small dingey or attempting to connect with an unfamiliar culture.