C ndido Rondon is by any measure the greatest tropical explorer in history. Between 1890 and 1930, he navigated scores of previously unmapped rivers, traversed untrodden mountain ranges, and hacked his way through jungles so inhospitable that even native peoples had avoided them--and led Theodore Roosevelt and his son, Kermit, on their celebrated "River of Doubt" journey in 1913-14. Upon leaving the Brazilian Army in 1930 with the rank of a two-star...