He blundered into the effect of phage on the mutation of bacterial species—very beautiful, very delicate—and after plodding months when he had been a sane citizen, an almost good husband, an excellent bridge-player, and a rotten workman, he knew again the happiness of high taut insanity.
There are no more uses of "mutation" in the book.
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Most mutations are harmful.
Radiation is less dangerous after child-bearing years since mutations are no longer cumulative to the next generation.