xlvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
signs of inferiority to those of the present day; the Labyrinthodonts 
cannot be placed below the living Salamander and Triton; the 
Devonian Ganoids are closely related to Polypterus and to Lepido- 
siren. 
But when we turn to the higher Vertebrata, the results of recent 
investigations, however we may sift and criticize them, seem to me 
to leave a clear balance in favour of the doctrine of the evolution of 
living forms one from another. In discussing this question, how- 
ever, it is very necessary to discriminate carefully between the 
different kinds of evidence from fossil remains which are brought 
forward in favour of evolution. 
Every fossil which takes an intermediate place between forms 
of life already known, may be said, so far as it 1s intermediate, to 
be evidence in favour of evoiution, inasmuch as it shows a possible 
road by which evolution may have taken place. But the mere dis- 
covery of such a form does not, in itself, prove that evolution took 
place by and through it, nor does it constitute more than presump- 
tive evidence in favour of evolution in general. Suppose A, B, C 
to be three forms, of which B is intermediate in structure between 
A and C. Then the doctrine of evolution offers four possible alter- 
natives. 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 modifi- 
cations of B; or A, B, and C may be independent modifications of 
some unknown D. Take the case of the Pigs, the Anoplotherude, 
and the Ruminants. The Anoplothervide are intermediate between 
the first. and the last; but this does not tell us whether the Rumi- 
nants have come from the Pigs, or the Pigs from Ruminants, or both 
from Anoplotheriide, or whether Pigs, Ruminants, and Anoplothe- 
ride 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. Ishould 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 dealig 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 Anoplothertwm is intermediate 
between the Pigs and the Ruminants—without either affirming, or 
denying, any direct genetic relation between the three forms in- 
volved. 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. 
