Page - 458 - in The Origin of Species

458 ORIGIN OF SPECIES But I must explain my meaning more fully. I believe that the arrangement of the groups within each class, in due sub- ordination and relation to each other, must be strictly genea- logical in order to be natural; but that the amount of differ- ence in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the 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 repre- sent 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 de- scendants to the present day, represented by the fifteen genera (a" to /^) on the uppermost horizontal line. Now all these modified descendants from a single species, are re- lated in blood or descent in the same degree ; they may meta- phorically 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 be- long 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 modi- fied descendants from A will have inherited something in common from their common parent, as will all the descend- ants from I ; so will it be with each subordinate branch of descendants, at each successive stage. If, however, we sup- pose any descendant of A, or of I, to have become so much