THE MODERN STUDY OF ZOOLOGY 21 



primary branches of the stem represent the off- 

 spring of this all-important ancestor ; the secondary 

 branches their offspring, and so on, each branch 

 denoting a generation. Some of the branches die 

 off, stop, and become extinct ; others persist and 

 thrive : the ultimate branchlets of these last bear 

 the leaves, which are the actually living representa- 

 tives of the family, on the topmost of which we 

 inscribe our own name. 



A genetic classification of the animal kingdom 

 takes a similar form ; the stem represents the 

 earliest and most primitive form of animal life, the 

 branches the several forms derived successively 

 from this and from one another, while the terminal 

 leaves represent the actually existing forms. If we 

 draw horizontal lines across our tree, these may be 

 taken to represent the different geological ages ; 

 then the branches cut by any one line or plane will 

 represent the life on the earth at that period. Some 

 of the branches never reach the top of the tree ; 

 these represent forms of life that attained their full 

 development in some of the earlier epochs and then 

 became extinct. 



Such a genetic classification is at once felt to be 

 the only really natural one, to be in fact an ideal 

 classification. It is simply an embodiment of fact, 

 not an expression of opinion : it is therefore ab- 

 solutely unassailable, and what is more, true for all 

 time. The recognition of the possibility of such a 

 classification is the distinguishing feature of modern 

 zoology, and the determination of such genetic 

 histories or phylogenies of the several groups of 



