Classification. 29 
order to see how far it lends itself to this new inter- 
pretation. 
The first thing that we have to observe about the 
nexus is, that it is a nexus—not a single Sine, 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 Linneus). 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 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. Ac- 
cording to this system, a short trunk may be taken 
