DEDUCTION. 123 



organs of the highest functional value are 

 under the constant and pressing necessity of 

 changing with changes in the environment; 

 while those of least functional value remain 

 undisturbed, and pass, little or not at all modi- 

 fied, from generation to generation. The same 

 explanation holds for the rule concerning the 

 importance of "aggregates of unimportant char- 

 acters " in determining the affinities of animals 

 and plants. 



In some parts of the natural system, there 

 are what are called "chains of affinities." A 

 group, instead of being broken up into well 

 separated sub-groups, consists of a chain in 

 which the adjacent parts are closely related, 

 but the more distant parts have comparatively 

 few points in common. It is impossible to 

 break up the group without violating the affini- 

 ties of adjacent parts, and it is difficult to define 

 it in such a way as to include the extremes. 

 The Crustacea furnish an example. What was 

 a special difficulty under the old views of clas- 

 sification is explained under the doctrine of 

 descent. 1 



One of the most important principles that 

 had been established empirically was the tree- 

 like arrangement of species and higher groups 



1 Origin of Species, p. 368. 



