while B is intermediate in structure between A and C. Then the doctrine of evolution offers four possible alternatives.  A may have become C by way of B; or C may have become A by way of B; or A and C may be independent modifications of B; or A, B, and C may be independent modifications of some unknown D. Take the case of the Pigs, the Anoplotheridae, and the Ruminants.  The Anoplotheridae are intermediate between the first and the last; but this does not tell us whether the Ruminants have come from the Pigs, or the Pigs from Ruminants, or both from Anoplotheridae, or whether Pigs, Ruminants, and Anoplotheridae alike may not have diverged from some common stock.

But if it can be shown that A, B, and C exhibit successive stages in the degree of modification, or specialization, 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 anyone 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 recognized as such, the number of such forms has steadily increased among the higher Mammalia.  Not only do we now know numerous intercalary forms 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, Hyaenictis and Ictitherium as intercalary, or, perhaps, linear types between the Viverridae and the Hyaenidae.