Living things are able to move and respire: they take in food, respond to stimuli (irritability), and reproduce their own kind, whereas non-living things are unable to perform all these activities. There is a great diversity among living organisms. They are alike in their general characteristics, but differ in their detailed characteristics because of specialization. The classification of these organisms on the basis of their specialization is the concern of the taxonomy. The present method of classifying organisms was originated by Carolus von Linnaeus, a Swedish naturalist of the early eighteenth century. The Linnaeus system of taxonomy has since become greatly elaborated and is now in universal use. It is based on the proposition that if certain organisms can be shown to have similar body construction they can be regarded as members of the same classification group. An evolutionary inference is also made in his classification. Thus, taxonomy deals with the structural makeup of organisms directly, and with their evolutionary histories indirectly. Two Kingdom system of classification is incomplete and is now replaced by the Five Kingdom system of classification.