INTRODUCTION 5 



numerous limbs. These difFerences — for greater than those be- 

 tween classes — are expressed by placing the backboned animals 

 in the phylum or sub-kingdom ChortJitta, the many-legged, 

 armoured forms in the phylum Arthropmla. Similarly, soft-bodied 

 animals with shells, such as 0)'sters and Snails, form the phjdum 

 MoUusca, Polypes and Jelly-fishes the phylum Cuchnitcmlu. And 

 finall}- the various phyla recognised by zoologists together con- 

 stitute 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 arbitrarilj'', 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 — Chord ATA. 

 Class — Mammalia. 



Order — Carnivora. 

 Family — Felidw. 

 Genus — Fclis. 



Species — F. domcstica. 



The object of systematic zoologists has always been to find a 

 7iah(,ral as opposed to an artificial classification of animals. 

 Good instances of artificial classification are the grouping of Bats 

 with Birds on the ground that they both possess wings, and of 

 Whales with Fishes on the gi'ound that they both possess fins and 

 live in the water. An equally good example of a natural classi- 

 fication is the grouping of both Bats and Whales under the head of 

 Mammalia because of their agreement, in all essential points of 

 anatomy, histology, and embryology, with the hairy quadrupeds 

 which form the bulk of that class. 



With the older zoologists the difficulty was to find some general 

 principle to guide them in their arrangement of animals — some 

 true criterion of classification. It was believed by all but a few 

 advanced thinkers that the individuals of each species of animal 

 were descended from a common ancestor, but that the original 

 progenitor of each species was totally unconnected with that of 

 every other, having, as Buffon puts it, " participated in the grace 

 of a distinct act of creation." To take an instance — all Wolves 

 were allowed to be descended from a pair of ancestral Wolves, and 

 all Jackals from a pair of ancestral Jackals, but the original pair in 

 each case was supposed to have come into being by a supernatural 



