CHAP. XIV.] CLASSIFICATION. 347 



different degrees of modification which they have undergone ; and 

 this is expressed by the forms being ranked under different genera, 

 .families, sections, or orders. The reader will best understand 

 what is meant, if he will take the trouble to refer to the diagram 

 in the fourth chapter. We will suppose the letters A to L to 

 represent 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 u to 2") 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 F 14 may be supposed 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. Nevertheless 

 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 representa- 

 tion of the groups, as here given in the diagram on a flat 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 not possible to represent in a 

 series, on a flat 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 



