The Corps of Discovery, headed by Meriwether Lewis and William Clark, was instructed by President Thomas Jefferson to describe the geography, ecology and cultures of the great unknown tract the United States purchased from France in 1803---the Louisiana Purchase, encompassing much of the present-day interior West---and seek a water route to the Pacific Ocean.
Lewis described 260 plants in his trip journals, a majority of them new to science. Among them were the blue camas, a plant whose edible tuber was immensely significant to tribes like the Nez Perce; and Lewis' monkeyflower, one of many plants and animals bearing the name of one captain or the other.
Lewis and Clark made the first scientific descriptions of many bird species. The Clark's nutcracker, a member of the jay family, is one of them. This remarkable bird caches pine seeds across broad areas to subsist upon in the winter.
Some of the more dramatic organisms encountered by the expedition were mammals, most famously the irascible grizzly bear---a brown bear subspecies first described by Lewis and Clark---that challenged the Corps first on the bottoms of the Missouri River. They also sent back to Jefferson a live black-tailed prairie dog---a "barking squirrel," as they called it.
Reptiles and Amphibians
Some cold-blooded firsts on the expedition included the Pacific tree frog, the prairie rattlesnake and the California newt.
Among the fish Lewis and Clark discovered for the Western world was the mountain sucker, an algae-eating bottom-dweller they encountered in present-day Montana.