CLASSIFICATION 239 



quite impossible to give definitions by which the several 

 members of the several groups could be distinguished 

 from their more immediate parents and descendants. Yet 

 the arrangement in the diagram would still hold good 

 and would be natural; for, on the pri::»ciple of inheri- 

 tance, all the forms descended, for instance, from A, 

 would have something in common. In a tree we can 

 distinguisli this or that branch, though at the actual fork 

 the two unite and blend together. We could not, as I 

 have said, de&io 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 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 toward 

 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 types belong. 



Finally, we have seen that natural selection, which 

 follows from the struggle for existence, and which almost 

 inevitably leads to extinction and divergence of character 

 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 spe- 

 cies, although they may have but few characters in com- 

 mon; we use descent in classing acknowledged varieties, 



