History of classification

  Just after Cuvier’s classification, the Frenchnaturalist Étienne GeoffroySaint-Hilaire outlined the importance of homologous structures. Homology is correspondencebetween features caused by continuity of information. Thus,a bird’s wing is homologous to a bat’s wing insofar as both areforelimbs, but they are not homologous as wings. Homologous structures need notresemble each other; for example, the three bones in the middle ear of humansare homologous to three bones in the jaw apparatus in fishes because thegenetic and developmental information controlling them has been continuousthrough evolutionary change.

  Before evolution was generallyaccepted, homologies among different animals, when they were recognized at all,were regarded as aspects of God’s pattern. Evolution provided a testableexplanation for homologies. By carefully tracing selected homologies, it hasbeen possible to show that previously proposed classifications establishedinappropriate relationships based solely on form or function, or both; forexample, the radial symmetry of starfishes isnot homologous to that of coelenterates (such as jellyfish).

  Protozoans were onceconsidered to be animals because they move and do not photosynthesize. Closerstudy has shown, though, that their movement is by means of nonmuscularstructures (cilia, flagella, or pseudopods) and thatphotosynthesis in them has often been lost and gained. Protozoans do not,therefore, form a natural group but with algae form a eukaryotic kingdomseparate from plants and animals, called Protista.

  Like plants and animals, fungi arose from protists and are nowaccorded a kingdom of their own.