INTRODUCTION 7 



than two pairs of limbs. They thus differ in some of the 

 most fundamental features of their organisation from such 

 animals as Crabs, Insects, Scorpions, and Centipedes, which 

 have colourless blood, a jointed external skeleton, and 

 numerous limbs. These differences far greater than 

 those between classes are expressed by placing the back- 

 boned animals in the phylum or sub-kingdom Chordata, 

 the many-legged armoured forms in the phylum Arthropoda. 

 Similarly, soft-bodied animals with shells, such as Oysters 

 and Snails, form the phylum Mollusca, Polypes and Jelly- 

 fishes the phylum Coelenterata. And finally the various 

 phyla recognised by zoologists together constitute the 

 kingdom Animalia. 



Thus the animal kingdom is divided into phyla, the phyla 

 into classes, the classes into orders, the orders into families, 

 the families into genera, and the genera into species, while 

 the species themselves are assemblages of individual animals 

 agreeing with one another in certain constant characters. 

 It will be seen that the individual is the only term in the 

 series which has a real existence : all the others are mere 

 groups formed, more or less arbitrarily, by man. 



To return to the animal originally selected as an example, 

 it will be seen that the zoological position of the Domestic 

 Cat is expressed as follows : 



Kingdom ANIMALIA. 

 Phylum CHORDATA. 

 Class MAMMALIA. 



Order CARNIVORA. 

 Family Felidce. 

 Genus Felis. 



Species F. domestica. 



The object of systematic zoologists has always been to 



