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of the impression of one。 It is wonderful enough that such a
perishable thing as a feather； and nothing more； should be
discovered； yet； for a long time； nothing was known of this bird
except its feather。 But by and by a solitary skeleton was
discovered which is now in the British Museum。 The skull of this
solitary specimen is unfortunately wanting； and it is therefore
uncertain whether the Archaeopteryx possessed teeth or
not。 But the remainder of the skeleton is so well preserved
as to leave no doubt respecting the main features of the animal；
which are very singular。 The feet are not only altogether bird…
like； but have the special characters of the feet of perching
birds； while the body had a clothing of true feathers。
Nevertheless； in some other respects； Archaeopteryx is
unlike a bird and like a reptile。 There is a long tail composed
of many vertebrae。 The structure of the wing differs in some
very remarkable respects from that which it presents in a true
bird。 In the latter； the end of the wing answers to the thumb
and two fingers of my hand； but the metacarpal bones； or those
which answer to the bones of the fingers which lie in the palm
of the hand； are fused together into one mass； and the whole
apparatus； except the last joints of the thumb； is bound up in a
sheath of integument； while the edge of the hand carries the
principal quill…feathers。 In the Archaeopteryx； the
upper…arm bone is like that of a bird； and the two bones of the
forearm are more or less like those of a bird； but the fingers
are not bound togetherthey are free。 What their number may
have been is uncertain； but several； if not all； of them were
terminated by strong curved claws； not like such as are
sometimes found in birds； but such as reptiles possess； so that；
in the Archaeopteryx； we have an animal which； to a
certain extent； occupies a midway place between a bird and a
reptile。 It is a bird so far as its foot and sundry other parts
of its skeleton are concerned； it is essentially and thoroughly
a bird by its feathers； but it is much more properly a reptile
in the fact that the region which represents the hand has
separate bones； with claws resembling those which terminate the
forelimb of a reptile。 Moreover； it has a long reptile…like tail
with a fringe of feathers on each side； while； in all true birds
hitherto known； the tail is relatively short； and the vertebrae
which constitute its skeleton are generally peculiarly modified。
Like the Anoplotherium and the Palaeotherium；
therefore； Archaeopteryx tends to fill up the interval
between groups which； in the existing world； are widely
separated； and to destroy the value of the definitions of
zoological groups based upon our knowledge of existing forms。
And such cases as these constitute evidence in favour of
evolution； in so far as they prove that； in former periods of
the world's history； there were animals which overstepped the
bounds of existing groups； and tended to merge them into larger
assemblages。 They show that animal organisation is more flexible
than our knowledge of recent forms might have led us to believe；
and that many structural permutations and combinations； of which
the present world gives us no indication； may nevertheless
have existed。
But it by no means follows； because the Palaeotherium has
much in common with the horse； on the one hand； and with the
rhinoceros on the other； that it is the intermediate form
through which rhinoceroses have passed to become horses； or
vice versa； on the contrary； any such supposition would
certainly be erroneous。 Nor do I think it likely that the
transition from the reptile to the bird has been effected by
such a form as Archaeopteryx。 And it is convenient to
distinguish these intermediate forms between two groups； which
do not represent the actual passage from the one group to the
other； as intercalary types； from those linear
types which； more or less approximately； indicate the nature of
the steps by which the transition from one group to the other
was effected。
I conceive that such linear forms； constituting a series of
natural gradations between the reptile and the bird； and
enabling us to understand the manner in which the reptilian has
been metamorphosed into the bird type； are really to be found
among a group of ancient and extinct terrestrial reptiles known
as the Ornithoscelida。 The remains of these animals occur
throughout the series of mesozoic formations； from the Trias to
the Chalk； and there are indications of their existence even in
the later Palaeozoic strata。
Most of these reptiles； at present known； are of great size；
some having attained a length of forty feet or perhaps more。
The majority resembled lizards and crocodiles in their general
form； and many of them were； like crocodiles； protected by an
armour of heavy bony plates。 But； in others； the hind limbs
elongate and the fore limbs shorten； until their relative
proportions approach those which are observed in the short…
winged； flightless； ostrich tribe among birds。
The skull is relatively light； and in some cases the jaws；
though bearing teeth； are beak…like at their extremities and
appear to have been enveloped in a horny sheath。 In the part of
the vertebral column which lies between the haunch bones and is
called the sacrum； a number of vertebrae may unite together into
one whole； and in this respect； as in some details of its
structure； the sacrum of these reptiles approaches that
of birds。
But it is in the structure of the pelvis and of the hind limb
that some of these ancient reptiles present the most remarkable
approximation to birds； and clearly indicate the way by which
the most specialised and characteristic features of the bird may
have been evolved from the corresponding parts of the reptile。
In Fig。 6； the pelvis and hind limbs of a crocodile； a three…
toed bird； and an ornithoscelidan are represented side by side；
and； for facility of comparison； in corresponding positions；
but it must be recollected that； while the position of the
bird's limb is natural； that of the crocodile is not so。 In the
bird； the thigh bone lies close to the body； and the metatarsal
bones of the foot （ii。； iii。； iv。； Fig。 6） are； ordinarily；
raised into a more or less vertical position； in the crocodile；
the thigh bone stands out at an angle from the body； and the
metatarsal bones （i。； ii。； iii。； iv。； Fig。 6） lie flat on the
ground。 Hence； in the crocodile； the body usually lies squat
between the legs； while； in the bird； it is raised upon the hind
legs； as upon pillars。
In the crocodile； the pelvis is obviously composed of three
bones on each side： the ilium （Il。）； the pubis
（Pb。）； and the ischium （Is。）。 In the adult bird
there appears to be but one bone on each side。 The examination
of the pelvis of a chick； however； shows that each half is made
up of three bones； which answer to those which remain distinct
throughout life in the crocodile。 There is； therefore； a
fundamental identity of plan in the construction of the pelvis
of both bird and reptile； though the difference in form；
relative size； and direction of the corresponding bones in the
two cases are very great。
But the most striking contrast between the two lies in the bones
of the leg and of that part of the foot termed the tarsus； which
follows upon the leg。 In the crocodile； the fibula （F） is
relatively large and its lower end is complete。 The tibia
（T） has no marked crest at its upper end； and its lower
end is narrow and not pulley…shaped。 There are two rows of
separate tarsal bones （As。； Ca。； &c。） and four distinct
metatarsal bones； with a rudiment of a fifth。
In the bird； the fibula is small and its lower end diminishes to
a point。 The tibia has a strong crest at its upper end and its
lower extremity passes into a broad pulley。 There seem at first
to be no tarsal bones； and only one bone； divided at the end
into three heads for the three toes which are attached to it；
appears in the place of the metatarsus。
In the young bird； however； the pulley…shaped apparent end of
the tibia is a distinct bone； which represents the bones marked
As。； Ca。； in the crocodile； while the apparently single
metatarsal bone consists of three bones； which early unite with
one another and with an additional bone； which represents the
lower row of bones in the tarsus of the crocodile。
In other words； it can be shown by the study of development that
the bird's pelvis and hind limb are simply extreme modifications
of the same fundamental plan as that upon which these parts are
modelled in reptiles。
On comparing the pelvis and hind limb of the ornithoscelidan
with that of the crocodile； on the one side； and that of the
bird；