But if it can be shown that A, B, and C exhibit successive stages in the degree of modification, or specialisation, of the same type; and if, further, it can be proved that they occur in successively newer deposits, A being in the oldest and C in the newest, then the intermediate character of B has quite another importance, and I should accept it, without hesitation, as a link in the genealogy of C. I should consider the burden of proof to be thrown upon any one who denied C to have been derived from A by way of B, or in some closely analogous fashion; for it is always probable that one may not hit upon the exact line of filiation, and, in dealing with fossils, may mistake uncles and nephews for fathers and sons.

I think it necessary to distinguish between the former and the latter classes of intermediate forms, as intercalary types and linear types.  When I apply the former term, I merely mean to say that as a matter of fact, the form B, so named, is intermediate between the others, in the sense in which the Anoplotherium is intermediate between the Pigs and the Ruminants—­without either affirming, or denying, any direct genetic relation between the three forms involved.  When I apply the latter term, on the other hand, I mean to express the opinion that the forms A, B, and C constitute a line of descent, and that B is thus part of the lineage of C.

From the time when Cuvier’s wonderful researches upon the extinct Mammals of the Paris gypsum first made intercalary types known, and caused them to be recognised as such, the number of such forms has steadily increased among the higher Mammalia.  Not only do we now know numerous intercalary forins of Ungulata, but M. Gaudry’s great monograph upon the fossils of Pikermi (which strikes me as one of the most perfect pieces of palaeontological work I have seen for a long time) shows us, among the Primates, Mesopithecus as an intercalary form between the Semnopitheci and the Macaci; and among the Carnivora, Hyoenictis and Ictitherium as intercalary, or, perhaps, linear types between the Viverridoe and the Hyoenidoe.

Hardly any order of the higher Mammalia stands so apparently separate and isolated from the rest as that of the Cetacea; though a careful consideration of the structure of the pinnipede Carnivora, or Seals, shows, in them, many an approximation towards the still more completely marine mammals.  The extinct Zeuglodon, however, presents us with an intercalary form between the type of the Seals and that of the Whales.  The skull of this great Eocene sea-monster, in fact, shows by the narrow and prolonged interorbital region; the extensive union of the parietal bones in a sagittal suture; the well-developed nasal bones; the distinct and large incisors implanted in premaxillary bones, which take a full share in bounding the fore part of the gape; the two-fanged molar teeth with triangular and serrated crowns, not exceeding five on each side in each jaw; and the existence of a deciduous dentition—­its close relation with the Seals.  While, on the other hand, the produced rostral form of the snout, the long symphysis, and the low coronary process of the mandible are approximations to the cetacean form of those parts.