230 AFFINITIES CONNECTING [Chap. XIV. 



whether large or small, and thus give a general idea 

 of the value of the differences between them. This is 

 what we should be driven to, if we were ever to succeed 

 in collecting all the forms in any one class which have 

 lived throughout all time and space. Assuredly we 

 shall never succeed in making so perfect a collection : 

 nevertheless, in certain classes, we are tending towards 

 this end ; and Milne Edwards has lately insisted, in an 

 able paper, on the high importance of looking to types, 

 whether or not we can separate and define the groups 

 to which such t}^es belong. 



Finally, we have seen that natural selection, which i 

 follows from the struggle for existence, and which almost 

 inevitably leads to extinction and divergence of charac- r 

 ter in the descendants from any one parent-species, 

 explains that great and universal feature in the affinities 

 of all organic beings, namely, their subordination in 

 group under group. We use the element of descent in ' 

 classing the individuals of both sexes and of all ages 

 under one species, although they may have but few 

 characters in common ; we use descent in classing ? 

 acknowledged varieties, however different they may be 

 from their parents ; and I believe that this element of ) 

 descent is the hidden bond of connexion which natur- 

 alists have sought under the term of the Natural System. 

 On this idea of the natural system being, in so far as it 

 has been perfected, genealogical in its arrangement, 

 with the grades of difference expressed by the terms 

 genera, families, orders, &c., we can understand the 

 rules which we are compelled to follow in our classifi- 

 cation. We can understand why we value certain 

 resemblances far more than others; why we use 

 rudimentary and useless organs, or others of trifling 



y 



