“I’ll give you my word we know nothing about it, professor,” declared Tom.  “Dick and I have been working all the morning unpacking stuff from the Wondership.”

The professor looked at them incredulously.

“That’s right,” struck in Zeb, “they haven’t been out of my sight.”

“But—­but,” stammered the professor, “my dear sir, that bag of specimens didn’t walk off, you know.  Besides,” he added, “I heard a human footfall distinctly.”

“It may not have been the boys, though,” spoke up Jack seriously.

“Indeed, who else then?” inquired the professor stiffly.

“An unwelcome neighbor,” replied Jack.  “We are not alone on this island.”

“Not alone?  What do you mean?” demanded the professor in thunderstruck tones.

“Just this, that there is someone else on it.  Who or what it is I don’t know.”

And Jack went on to explain all that he had seen.

CHAPTER XXXIII.

THE SECRET AT LAST.

Mysteries are always uncomfortable.  As Jack proceeded with his narrative, Dick and Tom looked nervously about them.  Even the boys’ two elders looked grave.  The presence of a man on the island was almost inexplicable.  But Jack’s story was so circumstantial that there was no room to suppose that he might be mistaken.  Besides, he had the bit of canvas to show, the scrap that he had taken from the thornbush.

After dinner Tom and Dick resumed their work of unloading necessaries from the Wondership.  Jack and the two elder members of the party discussed plans.

“You haven’t found any trace of mineral-bearing rock yet, have you, professor?” asked Jack.

The professor shook his head.

“Not a speck of anything that even remotely corresponds with the black sand that Zeb brought East with him,” said the man of science, dejectedly.

“It isn’t possible that we have been fooled,” said Zeb.

“Or landed on the wrong island,” struck in Jack.

“It must be the right island,” declared Zeb.

“How do you make that out?” asked Jack.

“Well, it’s got every mark on it that the map gives, for one thing,” said Zeb.

“That’s so,” agreed the professor, and then he added hopefully:  “However, I haven’t covered half the ground yet.”

Tom and Dick came tramping back at that juncture.  They carried some canned goods and Dick bore the rusty shovel that they had seen the day before sticking up in the black barren.

“It was sticky and moist out there,” he said, “but I figured we could always use this shovel, so I went out and brought it along.”

He flung himself down full length in the shade for it was hot and there was not a breath of wind to fan the canyon.  The professor, who sat facing Dick, concentrated his attention for an instant on the soles of the youngster’s boots.  Then he leaped up with a yell that startled them.