CLASSIFICATION A TREE-LIKE STRUCTURE. 59 



the animal kingdom ought to be arranged in parallel 

 lines. " Strict classification of forms supposed con- 

 stant excludes any natural relationship." If the 

 types were definite norms, their lowest representa- 

 tives should be just as different from animals of 

 other types as are the highest. Each animal would 

 be moulded strictly in accordance with its type, and 

 since the types are distinct, the different sub-king- 

 doms could not approach each other, but would 

 remain as separate parallel lines from their first ap- 

 pearance. And, moreover, since to make the theory 

 complete it is necessary to assume sub-types, the 

 same would be true of smaller divisions, and there 

 could be no conveyance of lines of relationship 

 toward each other, but they would remain always 

 parallel. If, e. g., the mammal and the fish are dis- 

 tinct creations according to different sub-types, they 

 ought to be as distinct when created as they are to- 

 day. But it was claimed by Darwin, and the claim 

 has been growing in favor since then, until it has 

 received universal acceptance, that such is not the 

 relation of the divisions of the animal kingdom. 

 There are no parallel lines, but all converge toward 

 each other as we go downward and backward. The 

 relation of the different groups of the animal and 

 vegetable kingdoms is to be looked upon in the 

 form of a branching tree, each sub-kingdom repre- 

 senting a great branch ; each class,order, family, and 

 genus smaller and smaller branches, while the species 

 are simply the terminal twigs and leaves. The 

 trunk of the tree, where all of the great branches 

 unite, has been carried farther and farther back until 



