"Well, M'Allister," I replied, "now you know for certain that upon a small planet gravitation really is much less than upon a larger planet of the same kind.
"That's another little wrinkle for you, and you have found it all out through tripping over a stone!"
"Losh, mon," replied he, "I seem to have learnt something almost every day since I have been here; even a tumble down teaches me something!"
I then drew his attention to the birds flying near us, and pointed out that they had a much wider spread of wing than our birds have, and that this was owing to the fact that the air being so thin a wide spread of wing was absolutely necessary to support them in the air and enable them to fly. I further explained that, if the gravitation upon Mars were as great as upon the earth, the birds' wings must necessarily have been still larger, as the pull of the planet would have been so much the greater, and would thus have prevented the birds from flying at all in such thin air if their wings had been small.
"M'Allister," I then remarked, "you will, no doubt, have noticed the same thing with regard to those large and beautiful butterflies 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.