“And the folk in the coal-mines,” said Summerlee with a dismal chuckle.  “If ever geologists should by any chance live upon earth again they will have some strange theories of the existence of man in carboniferous strata.”

“I don’t profess to know about such things,” remarked Lord John, “but it seems to me the earth will be `To let, empty,’ after this.  When once our human crowd is wiped off it, how will it ever get on again?”

“The world was empty before,” Challenger answered gravely.  “Under laws which in their inception are beyond and above us, it became peopled.  Why may the same process not happen again?”

“My dear Challenger, you can’t mean that?”

“I am not in the habit, Professor Summerlee, of saying things which I do not mean.  The observation is trivial.”  Out went the beard and down came the eyelids.

“Well, you lived an obstinate dogmatist, and you mean to die one,” said Summerlee sourly.

“And you, sir, have lived an unimaginative obstructionist and never can hope now to emerge from it.”

“Your worst critics will never accuse you of lacking imagination,” Summerlee retorted.

“Upon my word!” said Lord John.  “It would be like you if you used up our last gasp of oxygen in abusing each other.  What can it matter whether folk come back or not?  It surely won’t be in our time.”

“In that remark, sir, you betray your own very pronounced limitations,” said Challenger severely.  “The true scientific mind is not to be tied down by its own conditions of time and space.  It builds itself an observatory erected upon the border line of present, which separates the infinite past from the infinite future.  From this sure post it makes its sallies even to the beginning and to the end of all things.  As to death, the scientific mind dies at its post working in normal and methodic fashion to the end.  It disregards so petty a thing as its own physical dissolution as completely as it does all other limitations upon the plane of matter.  Am I right, Professor Summerlee?”

Summerlee grumbled an ungracious assent.

“With certain reservations, I agree,” said he.

“The ideal scientific mind,” continued Challenger—­“I put it in the third person rather than appear to be too self-complacent—­the ideal scientific mind should be capable of thinking out a point of abstract knowledge in the interval between its owner falling from a balloon and reaching the earth.  Men of this strong fibre are needed to form the conquerors of nature and the bodyguard of truth.”

“It strikes me nature’s on top this time,” said Lord John, looking out of the window.  “I’ve read some leadin’ articles about you gentlemen controllin’ her, but she’s gettin’ a bit of her own back.”