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 animals 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 an infinite number of minor differences 

 possible, 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 except 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 

 possibility of inheritance of such differentiations. 



Zoologists recognize some seventeen phyla or races of ani- 

 mals as follows : t 



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