ORGANIC BEINGS. 421 



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 descend- 

 ants. Yet the arrangement in the diagram would still hold 

 good and would be natural ; for, on the principle of inherit- 

 ance, 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 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 fol- 

 lows 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 or- 

 ganic 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 connec- 

 tion 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, orders, etc., we can under- 

 stand the rules which we are compelled to follow in our 



