to be more highly organized than a
zebra or an ass; although the entire horse-genus is clearly a more
highly organized type than any genus of animal which is not a mammal.

In view of these facts, therefore, the system of classification which
was eventually arrived at before the days of Darwin, was the system
which naturalists likened to a tree; and this is the system which all
naturalists now agreed upon as the true one. According to this system, a
short trunk may be taken 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 represent
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 belonging 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
engaged in tracing when they proceeded ever more and more accurately to
define these ramifications of natural affinity. But now, as just
remarked, we can clearly perceive that this underlying principle was
none other than Heredity as expressed in family likeness,--likeness,
the