4 NATURAL HISTORY. 



our domestic tabby, — a similar skeleton, teeth almost exactly alike, — so 

 cat, tiger, and lion are regarded as belonging to one genus ; and so on 

 through higher divisions — family, order, class, and branch in the order 



named. 



It must, however, be noted that classification, though professedly 

 natural, is in reality largely artificial, and that the apparent distinctions 

 between not only the different species of a genus, but also of the higher 

 groups, are but indicative of our ignorance ; for did we know all the 

 forms that exist or have existed, they could be so arranged that no one 

 could draw the line and say, here one genus ends, here a family begins. 

 We divide time into years, days, hours, minutes, and seconds, but this is 

 an arbitrary division : time, like the living world, forms an unbroken 

 series, becoming lost in eternity at either end. 



Proceeding upon the principles just outlined, most naturalists to-day 

 divide the whole animal kingdom into nine grand divisions or "branches," 

 — Protozoa, Sponges, Coelenterates, Echinoderms, Worms, Molluscs, Mol- 

 luscoidea, Arthropods, and Vertebrates. Concerning some of these some 

 doubt may exist. Thus, the sponges may possibly have to be united with 

 the coelenterates, while the groups of worms and of molluscoids may each 

 require to be broken up into two or more groups ; but this we can dismiss 

 without further mention, and can adopt the scheme just indicated. 





