It is the customary fate of new truths to begin as heresies and to end as superstitions.

Huxley is describing a recurring pattern in intellectual history: genuinely new ideas are first treated as “heresy” because they threaten established authorities, habits of thought, or institutional interests. Over time, if the idea proves useful or persuasive, it can become orthodox—so widely accepted that it hardens into “superstition,” i.e., a belief held by rote rather than by continued critical inquiry. The remark is also a warning to scientists and reformers: winning acceptance is not the end of the task; truths must be kept alive by evidence, debate, and openness to revision, or they risk becoming dogma indistinguishable from the errors they once displaced.