438 
NATURE 
[Fed. 24, 1879 
chalk, but that it remains, so to speak, in the possession of the 
ancestral estate ; and that from the cretaceous period (if not much 
earlier) to the present day, the deep sea has covered a large part 
of what is now the area of the Atlantic. But if Glodigerina, and 
Terebratula caput-serpentis and Beryx, not to mention other 
forms of animals and of plants, thus bridge over the interval 
between the present and the Mesozoic periods, is it possible that 
the majority of other living things underwent a ‘‘sea change into 
something new and strange” all at once ? 
7. Thus far I have endeavoured to expand, and to enforce by 
fresh arguments, but not to modify in any important respect, the 
ideas submitted to you on a former occasion. But when I come 
to the propositions touching progressive modification, it appears 
tome, with the help of the new light which has broken from 
various quarters, that there is much ground for softening the 
somewhat Brutus-like severity with which I have dealt with a 
doctrine, for the truth of which I should have been glad enough 
to be able to find a good foundation, in 1862. So far indeed as the 
Invertebrata and the lower Vertebrata are concerned, the facts and 
the conclusions which are to be drawn from them appear to me to 
remain what they were. For anything that, as yet, appears to the 
contrary, the earliest known Marsupials may have been as highly 
organised as their living congeners ; the Permian lizards show 
no signs of inferiority to those of the present day; the Laby- 
rinthodonts cannot be be placed below the living Salamander 
and Triton; the Devonian Ganoids are closely related to 
Polypterus and to Lepidosiren. 
But when we turn to the higher Vertebrata, the results of recent 
investigations, however we may sift and criticise 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, however, 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 such fossil which takes an intermediate place between 
forms of life already known, may be said, so far as it is inter- 
mediate, to be evidence in fayour of evolution, inasmuch as it 
shows a possible road by which evolution may have taken 
place. But the mere discovery of such a form does not, in 
itself, prove that evolution took place by and through it, nor 
does it constitute more than presumptive 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 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, andC may be independent modifications of some 
unknown D, Take the case of the Pigs, the Amoplotheride and 
the Ruminants. The 4xof/otheride are intermediate between the 
fiist and the last ; but this does not tell us whether Ruminants 
have come from the pigs, or pigs from RKuminants, or both 
from A xoplotheride, or whether pigs, Ruminants, and A zoflothe- 
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 specialisation, of the same 
type; and if, further, it can be proved that they occur in succes- 
sively 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 gene- 
alogy 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 zfercalary types 
and /inear types. When I apply the former term I merely mean 
to say, that asa matter of fact, the form B, so named, is interme- 
diate between the others, in the sense in which the A xop/otherium 
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 
forms of Ungzlata, but M. Gaudry’s great monograph upon the 
fossils of Pikermi (which strikes me as one of the most perfect 
pieces of palzeontological work I have seen for a long time) 
shows us, among the Primates, Mesopithecus as an intercalary form 
between the Semopitheci and the Macaci; and among the 
Carnivora, Hyenictis, and Ictitherium as intercalary, or, per- 
haps, linear, types between the /zverride and the Hyenide. 
Hardly any order of the higher Mammalia stands so appa- 
rently separate and isolated from the rest as that of the Cefacea, 
though a careful consideration of the structure of the fissipede 
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 ina 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. 
The scapula resembles that of the cetacean /yferoodon, but 
the supra-spinous fossa is larger and more seal-like ; as is the 
humerus, which differs from that of the Cefacea in presenting 
true articular surfaces for the free jointing of the bones of the 
fore-arm. In the apparently complete absence of hinder limbs, 
and in the characters of the vertebral column, the Zezg/odon lies 
on the cetacean side of the boundary line; so that, upon the 
whole, the Zeuglodonts, transitional as they are, are conveniently 
retained in the cetacean order. And the publication, in 1864, of 
M. Van Beneden’s memoir on the miocene and _ pliocene 
Sgualodon, furnished much better means than anatomists pre- 
viously possessed, of fitting in another link of the chain which 
connects the existing Cefacea with Zeuglodon. The teeth are 
much more numerous, although the molars exhibit the zenglodont 
double fang ; the nasal bones are very short, and the upper sur- 
face of the rostrum presents the groove, filled up during life by 
the prolongation of the ethmoidal cartilage, which is so charac- 
teristic of the majority of the Ce¢acea. 
It appears to me that, just as among the existing Carnivora, 
the walruses and the eared seals are intercalary forms between 
the fissipede Carnivora and the ordinary seals ; so the Zeuglodons 
are intercalary between the Carnivora, asa whole, and the Cetacea. 
Whether the Zeuglodonts are also linear types in their relation to 
these two groups cannot be ascertained, until we have more definite 
knowledge than we possess at present, respecting the relations in 
time of the Carvzivora and the Cetacea. 
Thus far, we have been concerned with the intercalary types 
which occupy the intervals between families, or orders, of the 
same class. But the investigations which have been carried on 
by Prof. Gegenbaur, Prof. Cope, and myself, into the structure 
and relations of the extinct reptilian forms of Dinosauria and 
Compsognatha, have brought to light the existence of intercalary 
forms between what have hitherto been always regarded as 
very distinct classes of the vertebrate sub-kingdom, namely, Rep- 
tilia and Aves. Whatever inferences may, or may not, be drawn 
from the fact, it is now an established truth that, in many of 
these Ornithoscelida, the hind limbs and the pelvis are much 
more similar to those of birds than they are to those of reptiles, 
and that these Bird-reptiles, or Reptile-birds, were more or less 
completely bipedal. 
When I addressed you in 1862, I should have been bold indeed 
had I suggested that paleontology would before long show us 
the possibility of a direct transition from the type of the lizard 
to that of the ostrich. At the present moment we have, in the 
Ornithoscelida, the intercalary type, which proves that transition 
to be something more than a possibility. But it is very doubtful 
whether any of the genera of Oy vithoscelida with which we are 
at present acquainted are the actual linear types by which the 
transition from the lizard to the bird was effected. These are, 
very probably, still hidden from us in the older formations. 
Let us now endeavour to find some cases of true linear types, 
or forms which are intermediate between others because they 
stand in a direct genetic relation to them. It is no easy matter 
to find clear and unmistakeable evidence of filiation among fossil 
