“Aye,” remarked Willis, “we are here to-day and gone to-morrow.”

“Suppose, then, that the population of the earth were twice as great, cultivation would be extended, territories that are now lying waste would be teeming with life and covered with fertile fields, but the same beautiful equilibrium would be maintained.”

“And the inhabitants of the planets,” said Fritz, “what are they about?”

“What planets do you mean?” inquired Willis.

“Well, all in general; the moon, for example, in particular.”

“The moon,” replied Jack, “has, in the first place, no atmosphere.  This we know, because the rays of the stars passing behind her are not, in the slightest degree, refracted; and this proves that neither men, nor animals, nor vegetables of any kind, are to be found in that planet, for they could not exist without air.”

“That should settle the question,” remarked Willis.

“Yes,” remarked Fritz; “but some theorists, nevertheless, insist that there may be living creatures in the moon, for all that—­of course, differently constituted from the inhabitants of our earth, and susceptible of existing without air.  There is, however, no evidence of any kind to support such a theory; it is a mere fancy, the dream of an imaginative brain.  Upon the same grounds, it may be argued, that the interior of the earth is inhabited, and that elves and gnomes are possible beings.  Besides, the telescope has been brought to so high a degree of perfection, that objects the size of a house can now be detected in the moon.”

“It seems, I am afraid,” remarked Jack, who, like his brother, was getting annoyed by the phantasmagoria on shore, “that we were about as well supplied with wild beasts here as they are with men in the planets.”

“In speaking of the moon, however,” continued Fritz, “I do not imply all the planets; for, certain as we are that the moon has no atmosphere, so we are equally certain that some of the planets possess that attribute.  Still there are other circumstances that render the notion of their being inhabited by beings like ourselves exceedingly improbable.  Mercury, for example, is so embarrassed by the solar rays, that lead must always be in a state of fusion, and water, if not reduced to a state of vapor, will be hot enough to boil the fish that are in it.  Uranus, at the other extremity of the system, receives four hundred times less heat and light than we do, consequently neither water nor any thing else can exist there in a liquid state; what is fluid on our earth must be frozen up into a solid mass.  Good, I declare my brother has fallen asleep!”

“It is very—­interesting—­however,” said Willis, making ineffectual efforts to smother a yawn.

“The same difficulty with comets; there must have been some very urgent necessity for human beings in order to have peopled them.  When they pass the perihelion—­”

“The what?” inquired Willis.