through many widely different forms.  If many extinct forms were to be discovered above one of the middle horizontal lines or geological formations—­for instance, above Number VI.—­but none from beneath this line, then only the two families on the left hand (namely, a14, etc., and b14, etc.) would have to be united into one family; and the two other families (namely, a14 to f14 now including five genera, and o14 to m14) would yet remain distinct.  These two families, however, would be less distinct from each other than they were before the discovery of the fossils.  If, for instance, we suppose the existing genera of the two families to differ from each other by a dozen characters, in this case the genera, at the early period marked VI., would differ by a lesser number of characters; for at this early stage of descent they have not diverged in character from the common progenitor of the order, nearly so much as they subsequently diverged.  Thus it comes that ancient and extinct genera are often in some slight degree intermediate in character between their modified descendants, or between their collateral relations.

In nature the case will be far more complicated than is represented in the diagram; for the groups will have been more numerous, they will have endured for extremely unequal lengths of time, and will have been modified in various degrees.  As we possess only the last volume of the geological record, and that in a very broken condition, we have no right to expect, except in very rare cases, to fill up wide intervals in the natural system, and thus unite distinct families or orders.  All that we have a right to expect, is that those groups, which have within known geological periods undergone much modification, should in the older formations make some slight approach to each other; so that the older members should differ less from each other in some of their characters than do the existing members of the same groups; and this by the concurrent evidence of our best palaeontologists seems frequently to be the case.

Thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the extinct forms of life to each other and to living forms, seem to me explained in a satisfactory manner.  And they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna of any great period in the earth’s history will be intermediate in general character between that which preceded and that which succeeded it.  Thus, the species which lived at the sixth great stage of descent in the diagram are the modified offspring of those which lived at the fifth stage, and are the parents of those which became still more modified at the seventh stage; hence they could hardly fail to be nearly intermediate in character between the forms of life above and below.  We must, however, allow for the entire extinction of some preceding forms, and for the coming in of quite new forms by immigration,