xlvili PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
intercalary between the Carnivora, as a 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 Carnivora and Cetacea. 
Thus far we have been concerned with the interealary types which 
oceupy the intervals between Families or Orders of the same class ; 
but the investigations which have been carried on by Prof. Gegen- 
baur, Prof. Cope, and myself into the structure and relations of the 
extinct reptilian forms of the Ornithoscelida (or 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 subkingdom, namely Reptilia 
and Aves. Whatever inferences may, or may not, be drawn from 
the fact, it is now an established truth that in many of these Orni- 
thoscelidu 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 Jong 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 Ornithoscelida 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, very probably, are 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 unmistakable evidence of filiation among tossil animals; for, in 
order that such evidence should be quite satisfactory, it is necessary 
that we should be acquainted with all the most important features 
of the organization of the animals which are supposed to be thus 
related, and not merely with the fragments upon which the genera 
and species of the paleontologist are so often based. M. Gaudry 
has arranged the species of Hyawnide, Proboscidea, Rhinocerotide, and 
Equide in their order of filiation from their earliest appearance in 
the Miocene epoch to the present time, and Professor Riitimeyer has 
drawn up similar schemes for the Oxen and other Ungulata—with 
what, I am disposed to think, is a fair and probable approximation to 
the order of nature. But, as no one is better aware than these two 
learned, acute, and philosophical biologists, all such arrangements 
must be regarded as provisional, except in those cases in which, by 
a fortunate accident, large series of remains are obtainable from a 
thick and wide-spread series of deposits. It is easy to accumulate 
probabilities—hard to make out some particular case in such a 
way that it will stand rigorous criticism. 
