Classification. 29 



order to see how far it lends itself to this new inter- 

 pretau>_n. 



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-hke 

 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 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 



