Synthesized answer
The provided passages do not contain information about the central thesis of "Essai sur la géographie des plantes." The passages only state the title of the work, the authors, and its categorization as Botany, along with its page count [1]. Therefore, it is not possible to answer what the central thesis of the text is based on this information.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Essai sur la géographie des plantes by Alexander von Humboldt, Aimé Bonpland Categories: Botany Pages: 168