Butterflies in America: To marvel at all things Butterfly People: An American Encounter with the Beauty of the World. By William Leach. Pantheon; 416 pages; $32.50. "BUTTERFLIES and butterflies," wrote Walt Whitman, "continue to flit to and fro, all sorts, white, yellow, brown, purple." Whitman may have had an eye for colour, but as a mere journalist and poet with perhaps a touch of lepidopterophilia, he does not quite qualify as one of William Leach's "Butterfly People." True butterfly people were far more serious. They were enthusiasts and obsessives who took advantage of the unfettered access to undeveloped land afforded by a young country pressing its frontier westward. They were artisans and aesthetes: scientists of a sort that gradually ceased to exist as the natural sciences grew more formal and mature. Mr Leach's compelling thesis is that 19th-century America provided a uniquely hospitable time and place for lovers, and especially collectors, of butterflies. Agricultural toil and westward expansion brought droves of ordinary people into intimate contact with vast expanses of pristine lands. Most 19th-century Americans, Mr Leach writes - uncharitably but probably accurately - saw nature "as something to dig up or harvest from the earth or to be torn down and transformed into real estate or money," and yet because there was so much untouched land, even the most rapacious miner or builder would in the course of his life come into contact with the natural world on a scale and in a way unknown to Americans today. Perhaps more important, "natural science" had not yet become "biology." It was practised out of doors rather than in a laboratory, and aside from the classification madness of Linnaeus, it was light on systems and formality. Its early titans, such as Charles Darwin and Alexander von Humboldt, a Prussian aristocrat whose masterwork, "The Cosmos," began being published in 1845, "seemed indifferent to drawing lines between art and science," Mr Leach writes. Their prose was often excellent and readable, so much so as to later seem unscientific. They often wrote on behalf of the artistic attractions of nature. They led with a sense of curiosity driven by rapture and wonder. Natural science belonged not to the trained and pedigreed biologist, but to the passionate amateur: they were "naturalists," not biologists. Butterfly people learned by observing, thinking and arguing. They included Samuel Scudder, who named the monarch butterfly and wrote a magisterial, three-volume work about North American butterflies that won raves from lepidoptery's literary champion, Vladimir Nabokov, 70 years after its publication. Another was William Henry Edwards. His great grandfather was America's most renowned Puritan theologian, and he eventually wrote his own three-volume doorstop. Mr Leach is at his best when profiling men such as Edwards, Scudder and others, including solitary, taciturn Herman Strecker and resentful, bilious Augustus Grote. These are the amateur giants on whose broad shoulders much contemporary biology and entomology rests.