Linnaeus based his early system of classification on the reproductive organs of plants, according to a history of Carl Linnaeus on the website of the University of California Museum of Paleontology. The class and order of a plant were decided by its stamens (male organs) and its pistils (female organs). Plants without obvious sexual orientation were lumped into the Class Cryptogamia, otherwise known as "plants with a hidden marriage."
Eventually Linnaeus developed a system that categorized living and inanimate things hierarchically, according to Education Portal. His system went from the broadest category to the most specific in descending order. By studying morphology, the appearance of a creature, Linnaeus classified more than 11,000 organisms. While modern science uses phylogenetics (evolutionary relationships) as a basis for organization, Linnaeus' work is still considered the foundation of taxonomy.
Charles Darwin's theory of evolution changed the way scientists categorized living creatures. While biologists agree with Linnaeus in placing pigs, porcupines and people in the genus Mammalia, they do it for different reasons, according to Understanding Evolution from the University of California Museum of Paleontology. Whereas Linnaeus sought to group species by appearance, modern taxonomists follow the evolutionary theory that the three species all had four legs and nursed their young because they descended from a common ancestor.