l flippers, the other kind reversed these attributes, their
heads being small, but their flippers long and powerful. They ran their
quarry down in the open, and seized them with outreaching tentacles. No
mistakes were made--no feints or false motions; and there was no
resistance by the victims. Where one was noticed he was doomed. The
tentacles gathered him in--to a murderous bite or a murderous embrace.

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