370 CLASSIFICATION. rCHAP. XIV. 



allied genera existing during the Silurian epoch, and descended from 

 some still earlier form. In three of these genera (A, F, and I), a 

 species has transmitted modified descendants to the present day, 

 represented by the fifteen genera (a 14 to z 14 ) on the uppermost 

 horizontal line. Now all these modified descendants from a single 

 species, are related in blood or descent in the same degree ; they 

 may metaphorically be called cousins to the same millionth degree ; 

 yet they differ widely and in different degrees from each other. The 

 forms descended from A, now broken up into two or three families, 

 constitute a distinct order from those descended from I, also broken 

 up into two families. Nor can the existing species, descended from 

 A, be ranked in the same genus with the parent A ; or those 

 from I, with the parent I. But the existing genus r 14 may be sup- 

 posed to have been but slightly modified ; and it will then rank with 

 the parent-genus F ; just as some few still living organisms belong 

 to Silurian genera. So that the comparative value of the differences 

 between these organic beings, which are all related to each other in 

 the same degree in blood, has come to be widely different. Never- 

 theless their genealogical arrangement remains strictly true, not 

 only at the present time, but at each successive period of descent. 

 All the modified descendants from A will have inherited something 

 in common from their common parent, as will all the descendants 

 from I ; so will it be with each subordinate branch of descendants, 

 at each successive stage. If, however, we suppose any descendant of 

 A, or of I, to have become so much modified as to have lost all traces 

 of its parentage, in this case, its place in the natural system will be 

 lost, as seems to have occurred with some few existing organisms. 

 All the descendants of the genus F, along its whole line of descent, 

 are supposed to have been but little modified, and they form a single 

 genus. But this genus, though much isolated, will still occupy its 

 proper intermediate position. The representation of the groups, 

 as here given in the diagram on a fiat surface, is much too simple. 

 The branches ought to have diverged in all directions. If the 

 names of the groups had been simply written down in a linear 

 series, the representation would have been still less natural ; and it 

 is notoriously net possible to represent in a series, on a fiat surface, 

 the affinities which we discover in nature amongst the beings of the 

 same group. Thus, the natural system is genealogical in its 

 arrangement, like a pedigree : but the amount of modification which 

 the different groups have undergone has to be expressed by ranking 

 them under different so-called genera, sub-families, families, sections, 

 orders, and classes. 

 It may be worth while to illustrate this view of classification, by 



