Something analogous occurs in grafting; for Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on another species, when thus grafted were rendered barren.
But if we look to varieties produced under nature, we are immediately involved in hopeless difficulties; for if two hitherto reputed varieties be found in any degree sterile together, they are at once ranked by most naturalists as species.
This may be illustrated by placing books edgeways on a carpet, and then, after the manner suggested by Michell, imitating the undulations of an earthquake: it will be found
that they fall with more or less readiness, according as their direction more or less nearly coincides with the line of the waves.
What the characters of the four sisters were we will leave to be found
Once on a hot day in August he tried but found
it stuck fast and after that he forgot all about it.