But some discussion of the complex relationship between “allohistory” and sf is appropriate here, as the genres overlap in certain ways. Classical allohistory— such as Trevelyan's What if Napoleon had won the Battle of Waterloo? and Churchill's If Lee had not won the Battle of Gettysburg —is a rigorously consistent thought-experiment in historical causality.

BEHIND him lay the gray Azores, / Behind the Gates of Hercules; / Before him not the ghost of shores, / Before him only shoreless seas. // The good mate said: “Now must we pray, / For lo! the very stars are gone. / Brave Admiral, speak, what shall I say?” / “Why, say, ‘Sail on! sail on! and on!’” “My men grow mutinous day by day; / My men grow ghastly wan and weak.” / The stout mate thought of home; a spray / Of salt wave washed his swarthy cheek. // “What shall I say, brave Admiral, say, / If we sight naught but seas at dawn?” / “Why, you shall say at break of day, / ‘Sail on! sail on! sail on! and on!’” They sailed and sailed, as winds might blow, / Until at last the blanched mate said: / “Why, now not even God would know / Should I and all my men fall dead. // These very winds forget their way, / For God from these dread seas is gone. / Now speak, brave Admiral, speak and say”— / He said: “Sail on! sail on! and on!” They sailed. They sailed. Then spake the mate: / “This mad sea shows his teeth to-night. / He curls his lip, he lies in wait, / With lifted teeth, as if to bite! // Brave Admiral, say but one good word: / What shall we do when hope is gone?” / The words leapt like a leaping sword: / “Sail on! sail on! sail on! and on!” Then, pale and worn, he kept his deck, / And peered through darkness. Ah, that night / Of all dark nights! And then a speck— / A light! A light! A light! A light! // It grew, a starlit flag unfurled! / It grew to be Time’s burst of dawn. / He gained a world; he gave that world / Its grandest lesson: “On! sail on!”

Lest that too heavenly form […] snare them.

Mr. Collinson then produced several samples of the apples; an untainted russeting, a russeting changed in complexion which grew among a great cluster of unaltered brethren, and some apples of the other tree which had caused the change in the russetings, and whose fruit had in turn received a rough cast from the russetings.