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 flat surface, is much too simple.