In the book The 21 Balloons, where are the twenty families on Krakatoa Island originally from? How did they get there?

In spite of the fact that each family represents the architecture and food of a different nation, all of the families are from the United States.  The "founding father" is Mr. M., a sailor who was washed up on the island after a shipwreck.  He gathered up some diamonds, built a raft, and steered himself towards the first ship he could see. The ship was on its way to San Francisco, where Mr. M. disembarked, sold some of his smaller diamonds, and then selected twenty families to take back with him to Krakatoa to settle the island. The families were subject to a few requirements: one boy and one girl, each somewhere between three and eight years of age, and an interest in some creative and/or scientific endeavor. Mr. M. thought that these were the most important qualifications to build a society and pass it on to subsequent generations.  Mr. M. bought a ship and trained everyone selected to become a good sailor.  They then sailed back to Krakatoa and began their lives there.  Nowhere in the book does it mention that Mr. M. must have found himself a wife, but there is a Mrs. M. and two children in the story, so we can safely assume that he must have gone wife-shopping in San Francisco.  There is no evidence of a conveniently ship-wrecked female having turned up for him.