The urge to classify living things is shared by scientists and non-scientists alike. Everyone can accept without too much prompting that, despite their obvious differences, eagles, ostriches and hummingbirds are all birds. Yet the major groups, or taxa, of living things are not simply a tidy way of organizing the millions of species that exist on Earth. The categories of life and their relationships in a connected hierarchy can be construed as a pared-down diagram of the very process of evolution.
Modern taxonomists wrestle con-Stanly with the best way of constructing a classification system that is meaningful and not arbitrary. What is it that makes, for example, the phylum Mollusca seem more “real” and less arbitrary as a group than, say, all those animals with red bodies? It is because the snails, slugs, squids, bivalve shellfish and other groups within the Mollusca share a common evolutionary history.