CHAPTER VII 

 HOMOLOGY AND THE BASIS OF CLASSIFICATION 



SYSTEMS of organs similar to those of the earthworm are found 

 in all animals higher in the zoological scale than Hydra and 

 the coelenterates. In some cases the organs are even simpler 

 than those of the earthworm, but in the great majority of ani- 

 mals they are more complex. The complexity is brought about 

 by the specialization of parts, leading to more extensive and 

 more detailed division of labor. The power of modification 

 possessed by animals makes possible an infinite number of minor 

 differences, as well as a great number of major differences, by 

 which we mean easily recognizable structural differences. A 

 species is a group of animals or plants in which the individuals 

 differ by no major structural differences. It is estimated that 

 more than 500,000 species of animals exist at the present time. 



Species of like nature are grouped into genera, or aggregates of 

 animal types, which agree in the main elements of structure 

 and function. Genera in turn are grouped into families, 

 families into orders, orders into classes, and classes into races or 

 phyla. Different phyla have few characters in common; 

 classes have numerous common features, orders still more, and 

 so on down to species in which all characters are similar, ex- 

 cept for minor variations, such minor features giving the basis 

 for varieties. 



One great interest to biologists in the study of comparative 

 anatomy is to trace out the relationships of parts which have 

 become differentiated from generalized organs. Another series 

 of problems has to do with the causes which have led to such 

 differentiation; and still another series has to do with the possi- 

 bility of inheritance of such differentiations. 



Zoologists recognize some seventeen primary phyla or races of 

 animals as follows: 



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