CLASSIFICATION A TREE-LIKE STRUCTURE. 6/ 



kingdom, it is necessary to show that the sub-king- 

 doms the types of Cuvier are themselves con- 

 nected at the bottom. This, we have seen, cannot 

 be done by living animals nor by fossils, but a third 

 source of evidence makes it possible. The subject 

 of embryology is also reserved for a later chapter. 

 The last few years' study in this subject has shown 

 that there is no lack of evidence that the great 

 types can be united in some common ancestor of 

 ancient times. Embryology enables us to connect 

 the different sub-kingdoms with each other at a com- 

 mon starting-point, and to trace this common form 

 still further back toward the simplest animal. And 

 thus our tree of classification has a trunk. If we 

 take any two animals and trace their embryology 

 backwards, we find them becoming more and more 

 alike, until they finally merge in a common form in 

 the early part of their history. 



And, finally, it must be noticed that not only do 

 the sub-kingdoms approach each other and unite at 

 a common point, but the two kingdoms of organic 

 life, animals and plants, in a like manner, converge. 

 So evident is this, that it has led to the establish- 

 ment of a third kingdom of organisms, neither 

 plants nor animals, but representing the union of 

 the two. This kingdom, called the Protista, con- 

 tains a number of microscopic forms. Although 

 the advisability of forming such a kingdom is very 

 questionable, the very fact that it has been done 

 by some naturalists is sufficient indication that the 

 animal and vegetable kingdoms do approach each 

 other at the bottom. 



