y 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 _Anoplothcridoe_, and the
Ruminants. The _Anoplothcridoe_ 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 _Anoplothcridoe_, or
whether Pigs, Ruminants, and _Anoplotlicridoe_ 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 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_