At last, when the inflow had ceased,—when there must have been millions of the brown killers in the tunnel,—the great hollow cylinder turned slowly on its axis and backed out through the rent in the wall, which immediately closed, with a crushing and scattering of fragments. Though the allies were far down-stream now, the war was practically ended; for the white defenders remained near the walls, and the black invaders were in wildest panic, each one, as the resistless current rushed him past, swimming against the stream, to put distance between himself and the destroyer below. But before long an advance-guard of the brown enemy shot out from the tributaries above, and the tide of retreat swung backward. Then came thousands of them, and the massacre was resumed. “Hot stuff, eh?” said his friendly neighbor to him. “Y-y-y-es—I guess so,” he answered, rather vacantly; “I don’t know. I don’t know anything about it. I never saw such doings. What is it all for? What does it mean?” “Oh, this is nothing; it’s all in a lifetime. Still, I admit it might ha’ been serious for us—and you, too—if we hadn’t got help.” “But who are they, and what? They all seem of a family, and are killing each other.” “Immortal shade of Darwin!” exclaimed the other sentry, who had not spoken before. “Where were you brought up? Don’t you know that variations from type are the deadliest enemies of the parent stock? These two brown breeds are the hundredth or two-hundredth cousins of the black kind. When they’ve killed off their common relative, and get to competing for grub, they’ll exterminate each other, and we’ll be rid of ’em all. Law of nature. Understand?” “Oh, y-yes, I understand, of course; but what did the black kind attack me for? And what do they want, anyway?” “To follow out their destiny, I s’pose. They’re the kind of folks who have missions. Reformers, we call ’em—who want to enforce their peculiar ideas and habits on other people. Sometimes we call them expansionists—fond of colonizing territory that doesn’t belong to them. They wanted to get through the cells to the lymph-passages, thence on to the brain and spinal marrow. Know what that means? Hydrophobia.” “What’s that?” “Oh, say, now! You’re too easy.”