organic world, which thus served to bind together organisms in groups
subordinate to groups--that is, into species, genera, orders, families,
classes, sub-kingdoms, and kingdoms. Third, that they were not able to
give any very intelligible reason for this faith that was in them;
sometimes supposing the principle in question to be that of a
supernatural plan of organization, sometimes regarding it as dependent
on conditions of physiology, and sometimes not attempting to account for
it at all.

Of course it is obvious that the theory of descent furnishes the
explanation which is required. For it is now evident to evolutionists,
that although these older naturalists did not know what they were doing
when they were tracing these lines of natural affinity, and thus helping
to construct a natural classification--I say it is now evident to
evolutionists that these naturalists were simply tracing the lines of
genetic relationship. The great principle pervading organic nature,
which was seen so mysteriously to bind the whole creation together as in
a nexus of organic affinity, is now easily understood as nothing more or
less than the principle of Heredity. Let us, therefore, look a little
more closely at the character of this network, in order to see how far
it lends itself to this new interpretation.

The first thing that we have to observe about the nexus is, that it is a
nexus--not a single line, or even a series of parallel lines. In other
words, some time before the theory of descent was seriously entertained,
naturalists for the most part had fully recognised that it was
impossible to arrange either plants or animals, with respect to their
mutual affinities, in a ladder-like series (as was supposed to be the
type of classification by the earlier systematists), or even in map-like
groups (as was supposed to be the type by Linnaeus). And similarly, also,
with respect to grades of organization. In the case of the larger
groups, indeed, it is usually possible to say that the members of this
group as a whole are more highly organized than the members of that
group as a whole; so that, for instance, we have no hesitation in
regarding the Vertebrata as more highly organized than the Invertebrata,
Birds than Reptiles, and so on. But when we proceed to smaller
subdivisions, such as genera and species, it is usually impossible to
say that the one type is more highly organized than another type. A
horse, for instance, cannot be said