d
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 a