378 THE ORIGIN OF SPECIES 



arrangement of the groups within each class, in due subordina- 

 tion and relation to each other, must be strictly genealogical in 

 order to be natural; but that the amount of difference in the 

 several branches or groups, though aUied in the same degree in 

 blood to their common progenitor, may differ greatly, being due 

 to the different degrees of modification which they have under- 

 gone; 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 

 and 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^^ to z^^) 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^* 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. Never- 

 theless, their genealogical arrangement remains strictly true, not 

 only at the present time, but at each successive period of de- 

 scent. All the modified descendants from A will have inherited 

 something in common from their common parents, 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 



