OHAP. XIV.] ORGANIC BEINGS. 357 



linked together by a long and only partially broken chain of 

 affinities. Extinction has only denned the groups : it has by no 

 means made them ; for if every form which has ever lived on 

 this earth were suddenly to reappear, though it would be quite 

 impossible to give definitions by which each group could be 

 distinguished, still a natural classification, or at least a natural 

 arrangement, would be possible. We shall see this by turning to 

 the diagram ; the letters, A to L, may represent eleven Silurian 

 genera, some of which have produced large groups of modified 

 descendants, with every link in each branch and sub-branch still 

 alive; and the links not greater than those between existing 

 varieties. In this case it would be 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 principle of inheritance, all 

 the forms descended, for instance, from A, would have something 

 in common. In a tree we can distinguish 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 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 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 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 naturalists 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, 



