tterflies we have
seen. Why, the outspread wings of the largest must have measured ten or
twelve inches across, and many of the smaller varieties were more than
six inches across. I wonder what our naturalists would say if they could
see some specimens of these large and splendidly coloured insects!"

"Well, Professor," he answered, "I never saw such large butterflies
anywhere else, not even when I was in the tropics on our own world. It
had never occurred to me that gravitation, or even the density of the
air, had anything to do with their size. Even now I do not understand
how it is the small insects are able to fly, for they are heavy for
their size, and do not possess very large wings, yet they can move very
swiftly."

"Let me explain then," I answered. "Large birds can only move their
wings with comparative slowness, and it is therefore necessary that
their wings should be large to enable them to keep their balance and be
able to fly. Their wings are somewhat in the nature of aeroplanes, and
they shift them to different angles to take advantage of the varying
currents of air.

"In the case of humming-birds and small insects, the wings are capable
of intensely rapid vibrations, so rapid indeed that, when flying, the
wings are almost, if not quite, invisible. This intensely rapid movement
enables them to fly, and is somewhat analogous to the rapid movements of
the vertical spiral screws, which you have seen on some of the Martian
air-ships that screw their way up into the air.

"Such rapid movements would not be suited to larger creatures, because
their muscular powers would have to be so enormously great that their
bodies would require to be larger and heavier in proportion. They would
thus be very unwieldy."

CHAPTER XXIII

I HAVE A SERIOUS TALK WITH JOHN

For some days past it had been becoming more and more evident to me that
John was quite infatuated with Siloni, and also that she was not
unwilling to receive his attentions. I could, therefore, no longer
remain a silent spectator, so took the first opportunity of our being
alone to broach the subject to him.

I began by saying, "John, have you any idea of remaining upon this
planet for the rest of your life?"

He looked round at me and flushed up. Then, after a little hesitation,
said, "No, Professor; why do you ask such a question as that?"

"Because, John," I answered, "it seems to me a very necessary question
to ask. If you are going away