Let us see how far these several facts and inferences accord with the theory of descent with modification.  As the subject is somewhat complex, I must request the reader to turn to the diagram in the fourth chapter.  We may suppose that the numbered letters represent genera, and the dotted lines diverging from them the species in each genus.  The diagram is much too simple, too few genera and too few species being given, but this is unimportant for us.  The horizontal lines may represent successive geological formations, and all the forms beneath the uppermost line may be considered as extinct.  The three existing genera, a14, q14, p14, will form a small family; b14 and f14 a closely allied family or sub-family; and o14, e14, m14, a third family.  These three families, together with the many extinct genera on the several lines of descent diverging from the parent-form A, will form an order; for all will have inherited something in common from their ancient and common progenitor.  On the principle of the continued tendency to divergence of character, which was formerly illustrated by this diagram, the more recent any form is, the more it will generally differ from its ancient progenitor.  Hence we can understand the rule that the most ancient fossils differ most from existing forms.  We must not, however, assume that divergence of character is a necessary contingency; it depends solely on the descendants from a species being thus enabled to seize on many and different places in the economy of nature.  Therefore it is quite possible, as we have seen in the case of some Silurian forms, that a species might go on being slightly modified in relation to its slightly altered conditions of life, and yet retain throughout a vast period the same general characteristics.  This is represented in the diagram by the letter F14.

All the many forms, extinct and recent, descended from A, make, as before remarked, one order; and this order, from the continued effects of extinction and divergence of character, has become divided into several sub-families and families, some of which are supposed to have perished at different periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms, supposed to be embedded in the successive formations, were discovered at several points low down in the series, the three existing families on the uppermost line would be rendered less distinct from each other.  If, for instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred, these three families would be so closely linked together that they probably would have to be united into one great family, in nearly the same manner as has occurred with ruminants and pachyderms.  Yet he who objected to call the extinct genera, which thus linked the living genera of three families together, intermediate in character, would be justified, as they are intermediate, not directly, but only by a long and circuitous course