144 SYSTEMATIC ZOOLOGY. 



used, the generic and the specific. Thus the cats all 

 belong to the genus Felis, and we have correspondingly 

 Felis leo, F. tigris, F. pardalis, F. domesticus, etc., for the 

 forms enumerated above. The specific name may be used 

 again and again in different genera, but the generic name 

 can be used but once in the animal kingdom. This use 

 of generic and specific names was introduced by Linne, 

 and was his greatest contribution to science. It must be 

 remembered, however, that these names are but aids in 

 the bookkeeping of zoological science. 



Animals may be classified in various ways, but that 

 universally adopted is based upon structure. Hence the 

 study, of anatomy is all important as a foundation for 

 this work. It therefore becomes necessary to distinguish 

 between two kinds of resemblance, that of analogy and 

 that of homology. Analogy is a resemblance in function 

 and not necessarily in structure; homology is based on 

 structure without necessarily similarity in function. Thus 

 the wings of a bird and of a butterfly are analogous in 

 that both are organs of flight, but they are totally different 

 in structure. The wing of a bird and the arm of man 

 are homologous, since the structure is essentially the same 

 in each, while their functions are different. The wing of 

 a bat and that of a bird are both analogous and homol- 

 ogous. 



PHYLUM I. PROTOZOA. 



The animal kingdom is divided into two great groups, 

 the Metazoa, in which the body is made up of many cells, 

 and the Protozoa, which mav be defined as animals each 



*/ 



consisting of a single cell. A little thought will show that 

 this difference is in reality very great. In the Metazoa 



