210 THE PHILOSOPHY OF BIOLOGY 



axial rod — the notochord — developing to form a 

 vertebral column ; and this notochordal skeleton is 

 also possessed by the Birds, Reptiles, Amphibia, and 

 Fishes. There are also some smaller groups in which 

 the notochord is present but does not develop to form 

 segmented vertebrae. Including these, we are able 

 to form a large category of animals — the Chordata — 

 and this phylum is sharply distinguished from all 

 other cognate groups. 



All animals and plants may be classified in a similar 

 way. Insects, Spiders, and Crustacea, for instance, 

 are all animals in which the body is jointed, each joint 

 or segment being typically provided with a pair of 

 jointed appendages or limbs. Because of this simi- 

 larity of fundamental structure we include all these 

 animals, with some others, in one phylum, the Arthro- 

 poda. So also with the rest of the animal kingdom, 

 and similar methods may be extended to the Iclassi- 

 iication of the plants. A few small groups in each of 

 the kingdoms are difficult to classify, but it has 

 been possible to arrange m.ost living organisms in a 

 small number of sub-kingdoms or phyla, and even to 

 attempt to trace relationships between these various 

 categories. 



The mere systematic description of the organic 

 world would have resulted in such a reasoned classifi- 

 cation apart altogether from any notions of an evolu- 

 tionary process. But the classification, originally a 

 conventional way of making a list of organisms, would 

 at once suggest morphological similarities. It would 

 suggest that all the Cats were Carnivores, that all the 

 Carnivores were Mammals, and that all the Mammals 

 were Chordates. It would suggest that all Wasps 

 were Hymenoptera, that all Hymenoptera were Insects, 

 and that all Insects were Arthropods. It would 



