a0 Darwin, and after Darwin. 
to represent the lowest organisms which cannot 
properly be termed cither 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 
