What does the last word of the text, indispensable, mean?

Absolutely necessary

Explanation:

Absolutely necessary I’d the correct answer. I’m the first part of the sentence, the author says, that “ it is a general law of nature that no organic being fertilizes itself for a perpetuity of generations.”

third person

explanation:

note the usage of names and pronouns, "james" & "the old man". this alone can confirm it as third person, as first person uses "i" and the like, while second person uses "you" and the like.

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its a hard situation i cant really answer because the essay is for you to write.