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Recognitions by contemporaries

Simón Bolívar wrote that "The real discoverer of South America was Humboldt, since his work was more useful for our people than the work of all conquerors". Charles Darwin expressed his debt to Humboldt, and admiration for his work, writing to Joseph Dalton Hooker that Humboldt was the " greatest scientific traveller who ever lived ". Wilhelm von Humboldt wrote that "Alexander is destined to combine ideas and follow chains of thoughts which would otherwise have remained unknown for ages. His depth, his sharp mind and his incredible speed are a rare combination." Johann Wolfgang Goethe observed that "Humboldt showers us with true treasures". Friedrich Schiller wrote that "Alexander impresses many, particularly when compared to his brother—because he shows off more!" José de la Luz y Caballero wrote that "Columbus gave Europe a New World; Humboldt made it known in its physical, material, intellectual, and moral aspects".

Napoléon Bonaparte remarked "You have been studying Botanics? Just like my wife!" Claude Louis Berthollet said "This man is as knowledgeable as a whole academy". Thomas Jefferson remarked "I consider him the most important scientist whom I have met". Emil du Bois-Reymond wrote that "Every assiduous scholar... is Humboldt's son; we are all his family." Robert G. Ingersoll wrote that "He was to science what Shakespeare was to the drama".

Hermann von Helmholtz wrote that "During the first half of the present century we had an Alexander von Humboldt, who was able to scan the scientific knowledge of his time in its details, and to bring it within one vast generalization. At the present juncture, it is obviously very doubtful whether this task could be accomplished in a similar way, even by a mind with gifts so peculiarly suited for the purpose as Humboldt's was, and if all his time and work were devoted to the purpose."