TH^ CLASSIFICATION OF ANIMALS. 119 



the dififerences of structure whicli separate these nine 

 groups are not equally great. The relation of the 

 different groups to one another can never be accu- 

 rately expressed by means of a tabular classification ; 

 it can only be understood by reference to the illustra- 

 tion that has already been used in speaking of the 

 distinction between plants and animals, — namely, that 

 of a branching tree. We may think of the Family 

 Tree of animal life as possessing a main trunk made 

 up of animal types which grow more complex as the 

 trunk rises, and of our nine divisions as having 

 branched off, some quite close to tbe older stock of 

 very simple types, others very high up, from the 

 newer stock of very complex types. Of the various 

 branches some have, and some havei not, sprouted off 

 very near to one another ; and if tliere stands between 

 two contiguous branches a little twig, belonging to 

 neither, but intermediate in position between the two, 

 this twig may be taken to represent the position of 

 any animal which forms a link between two groups. 

 The diagram on p. 305 will serve to illustrate this 

 analogy, and to indicate roughly the relationship of 

 the chief groups of the animal kingdom.^ 



^ It must of course be understood tliat this conventional " 

 representation of the relations of the various groups of the ■ 

 animal kingdom, although more correct than the representa- 

 tion of them by a tabular series, is still not wholly adequate 

 to express the very complicated ideas involved. It m.ust bo 

 added that the details of any such diagram are necessarily 

 to some extent provisional, and that future theories or dis- ; 



