CLASSIFICATION 389 



forms descended from A, or from I, would have some- 

 thing in common. In a tree we can specify this or that 

 branch, though at the actual fork the two unite and blend 

 together. We could not, as I have said, define the 

 several groups ; but we could pick out types, or forms, 

 representing most of the characters of each group, 

 whether large or small, and thus give a general idea of 

 the value of the diiferences between them. This is what 

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

 collecting all the forms in any class which have lived 

 throughout all time and space. We shall certainly 

 never succeed in making so perfect a collection : never- 

 theless, in certain classes, we are tending in this 

 direction ; 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 types belong. 



Finally, we have seen that natural selection, which 

 results from the struggle for existence, and which 

 almost inevitably induces extinction and divergence of 

 character in the many descendants from one dominant 

 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, although having few characters in 

 common, under one species ; we use descent in classing 

 acknowledged varieties, however different they may 

 be from their parent ; and 1 believe this element of 

 descent is the hidden bond of connection which natural- 

 ists 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 between the descendants from 

 a common parent, expressed by the terms genera, 

 families, orders, etc., we can understand the rules 

 which we are compelled to follow in our classification. 

 We can understand why we value certain resemblances 

 far more than others ; why we are permitted to use 

 rudimentary and useless organs, or others of trifling 



