30 Darwin, and after Darwin. 



to represent the lowest organisms which cannot 

 properly be termed either plants or animals. This 

 short trunk soon separates into two large trunks, one 

 of which represents the vegetable and the other the 

 animal kingdom. Each of these trunks then gives off 

 large branches signifying classes, and these give off 

 smaller, but more numerous branches, signifying 

 families, which ramify again into orders, genera, and 

 finally into the leaves, which may be taken to repre- 

 sent species. Now, in such a representative tree of 

 life, the height of any branch from the ground may be 

 taken to indicate the grade of organization which the 

 leaves, or species, present ; so that, if we picture to 

 ourselves such a tree, we may understand that while 

 there is a general advance of organization from below 

 upwards, there are many deviations in this respect. 

 Sometimes leaves growing on the same branch are 

 growing at a different level — especially, of course, if 

 the branch be a large one, corresponding to a class or 

 sub-kingdom. And sometimes leaves growing on 

 different branches are growing at the same level : 

 that is to say, although they represent species be- 

 longing to widely divergent families, orders, or even 

 classes, it cannot be said that the one species is more 

 highly organized than the other. 



Now, this tree-like arrangement of species in nature 

 is an arrangement for which Darwin is not responsible. 

 For, as we have seen, the detecting of it has been 

 due to the progressive work of naturalists for centuries 

 past ; and even when it was detected, at about the 

 commencement of the present century, naturalists 

 were confessedly unable to explain the reason of it, 

 or what was the underlying principle that they were 



