58 THE ARRAY OF LIVING ORGANISMS 



Exercises XII and Xll! 



TABLE 1 



now alive has as long a history of evolution as 

 any other — no contemporary organism is the 

 ancestor of any other contemporary organism — 

 certain groups have changed relatively little over 

 long periods of time. From these, and from 

 genuine ancestors preserved as fossils, it is pos- 

 sible to construct a genealogy of living things, 

 a tree of life, that shows the lines of ancestry 

 and divergence among living forms. On such a 

 tree, all present-day organisms have equivalent 

 status, at the tips of branches. It is the stems 

 and branch points that express their evolution. 



A first approach: the three kingdoms 



All living things may be divided into three 

 great kingdoms: plants, animals, and protists: 



Plants 

 Seed plants 

 Ferns 

 Mosses, liverworts 



Animals 

 Vertebrates 

 Invertebrates 

 Sponges, jellyfish 



Protists 



Plant-like 



Animal-like 



Blue-green Algae Slime molds Protozoa 



algae 

 Bacteria Fungi 



The protists are mainly single-celled organ- 

 isms, sometimes containing many nuclei within 



a single cell membrane (multinucleate). Some 

 are multicellular, but then display little or no 

 differentiation of tissues to perform specific 

 functions. We regard such aggregated protists 

 as colonial, to distinguish them from the multi- 

 tissued organisms. Usually the cells of such 

 colonial forms can also live independently, and 

 can give rise by division to new colonies. Most 

 protists are small; but red or brown seaweeds 

 may achieve great size and very complicated 

 shapes — giant kelps may be 150 feet long — 

 yet with little differentiation of tissues. (Vari- 

 ous authors include different groups within 

 the protists. Simpson, et al., include the proto- 

 zoa among them, but place the algae among the 

 plants. Weisz does not use the category protist 

 at all, and puts the animal-like protists among 

 the animals and the plant-like protists among 

 the plants. Stanier, et al., in The Microbial 

 World group them as here, and provide an ex- 

 cellent discussion of the relations among them.) 

 Fortunately most of you have good, working 

 notions of animals and plants, and could prob- 

 ably decide fairly accurately into which group 

 to place even unfamiliar forms. The character- 

 istics that divide these two kingdoms are sum- 

 marized in Table 1. 



Orders of classification 



The members of each kingdom are arranged 

 in a hierarchy of groupings. The major groups 



