Plants and animals are classified according to their resemblances and they are placed in one or other of a not very large number of groups such as ferns, conifers, molluscs, or mammals. But within each of these groups, there is subdivision into other smaller groups, mammals being so subdivided into rodents, carnivores, ungulates, and primates for example. Within these again there is further subdivision, and the important point to notice is that Classification always places species in groups that are contained within other larger groups. This is so common that its significance is often overlooked. Why do organisms have to be classified like this ? Why are they not strewn in single file up the ladder of the plant and animal kingdoms, or fortuitously like pebbles on a beach, or arbitrarily like the stars imaginary constellations ? The reason is that the arrangement of groups within groups is a natural classification reflecting the course of evolution. It is the result of descent from common ancestors and indication of affinity; the differences between the groups are due modification and divergence during such descent.

In a group classified in accordance with a natural system of classification it is possible to show the affinities and derivations of the various sub-groups in the form of a diagram or evolutionary tree. The resemblance in structure between different living organisms indicates the degree to which they are related, which is the basis of biological classification. Man is a warm-blooded, air-breathing animal with hairy skin, whose young are born alive and fed at the breast. He is therefore a mammal.

When Man's bodily structure is compared in detail with that of other mammals, it is clear that he belongs to the order of Primates, distinguished by large brains, and grasping hands and fingers with nails rather than claws. The Primates also include tree-shrews, lemurs, tarsiers, monkeys, and apes. Thomas Henry Huxley in one of his essays on Mans Place in Nature (1894), wrote: 'Whatever system of organs be studied ... the structural differences which separate Man from the Gorilla and Chimpanzee are not so great as those which separate the Gorilla from the lower apes' (i.e. monkeys).

Much of the work of eighteenth-century biologists concerned arranging species into taxonomies. Part of the urgency of this project arose from the sheer number of species discovered: in antiquity, Theophrastus could identify five hundred plant species; by the late Renaissance, Bauhin could identify over six thousand; Linnaeus catalogued eighteen thousand; and Cuvier  listed over fifty thousand separate species of plants. Although most earlier botanists had been content merely to describe individual species, natural philosophers of the late seventeenth and early eighteenth centuries began to see a need to arrange them into meaningful categories. Newton's classification of the heavenly bodies in Principia Mathematica (1687) increased the taxonomic urge in biologists at the beginning of the eighteenth century.

The classification of species, however, began well before the eighteenth century. Aristotle distinguished species by habitat and means of reproduction, but Andrea Cesalpino produced the first significant taxonomy of plants in 1583, arranging the species in a hierarchical, graded order. His work was developed by Marcello Malpighi, who expanded his hierarchical system to include animals.

The English naturalist John Ray was the first to formulate the idea of species. His late seventeenth- century work is based on the taxonomy of Aristotle, but he provided a sounder scientific basis on which to make distinctions of various plants and animals from one another.

The single most important development in taxonomy, however, came from the Swedish botanist Linnaeus, who in 1737 superseded Ray by publishing the taxonomic system which is the basis of that used today. Whereas most previous taxonomies worked by beginning with large categories subdivided along logical lines, Linnaeus worked in the opposite direction, beginning empirically with individual species and grouping them according to their similarities. He borrowed Bauhin's system of nomenclature, and identified twenty-four classes of plants, categorizing them by the number of pistils and stamens: this classification based on reproductive organs soon became the standard system. Later he identified six classes of animals (quadrupeds, birds, amphibians, fishes, insects, and worms), following the work of Ray.

Although Linnaeus established the basis of taxonomy still in use today, those who followed him refined and corrected his sytem. Among the most important was Jean- Baptise Lamarck, who was unsatisfied with Linnaeus's classification of invertebrates into only two classes, insects and worms. Lamarck distinguished mollusks, arthropods, crustaceans, insects, and other classes.

The classification of species carried with it considerable political and theological baggage. Voltaire, for instance, likened the hierarchical arrangement of species to political and religious hierarchies in the Philosophical Dictionary (1764). The debate became only more heated as taxonomy became entangled in arguments over evolution, as biologists began to explain both the similarities and differences of the species by placing them at different points along an evolutionary path. Early attempts to reconcile taxonomy with evolution were tentative and inconsistent: lacking an adequate empirical record (such as fossils), many theorists made wild speculations about the relationships between the species. But as the nineteenth century progressed and more evidence was collected, the gaps were filled and scientists reconciled the theory and the evidence without violence to either.