A History 
of Birds. 
(43) 
THE BIRD WORLD. 
neck by two such bosses. In the form of the 
backbone, and of the hip girdle, and in the 
structure of the legs, birds also agree with 
the reptiles. To' state the evidence on which 
these conclusions are founded, in full, would be 
wearisome to those who are not particularly 
interested in dry bones, but we may indicate 
the nature of the argument by a comparison 
of the hind-limb of the bird and reptile. 
COMPARISONS WITH REPTILES. 
This limb then, in both, is peculiar in that 
the ankle-joint is formed in such a way that 
when the foot is bent the joint turns on a hinge 
formed between two rows of ankle bones, while 
in the mammals the joint is formed by the hing¬ 
ing of the shank of the leg upon the uppermost 
row of ankle bones. But the bird’s leg, we may 
remark, is peculiar in that these two rows of 
ankle bones have undergone great modifications 
and can be seen in their originally separate con¬ 
dition only by examining the chick some time 
before hatching, though traces yet remain in, 
say, a young fowl of three months old. (See 
Fig. 3.) It is owing to the fact that these 
peculiarities are not generally known that most 
books on birds are inaccurate when they describe 
the “ legs and toes ” of a bird, the “ legs ” being 
the long “cannon-bones,” which are clad in 
scales and often brightly coloured. 
A reference to the accompanying diagram 
should make this clear. Here we have the leg 
of a fowl. The first joint is formed by the 
hinging of the femur, or thigh bone, marked 
F, with the tibia marked T, thus forming the 
“ knee ” joint. At the end of this tibia the joint 
with the ankle-bones takes place. But in the 
birds, as we have said, the two rows which 
these small bones make up disappear before 
adult life is reached. One row (marked Ai, 
Fig- 3) composed, in the half-grown bird, of 
a mallet-like piece, the handle of which runs up 
the front of the tibia or shank, becomes welded 
to the “ shank,” while the other (marked A2, Fig. 
3), composed of several small pieces originally, 
but now forming a thin plate, becomes welded 
on to the top of the bones of the foot. It is these 
last which are always, but wrongly, described 
as the “leg” bones. Really, they answer to 
the bones of our own feet which lie between the 
ankle and the toes. Finally, we have the toes, 
about which there can be no mistake. 
The great lengthening of the foot-bones has 
been brought about by the evolution of the 
bird from a crawling to a walking animal. 
Originally they were five in number, but are 
now, like the toes, reduced to four. Of these 
four, three are now welded together to form a 
single “ cannon ” bone, answering to that of 
the horse, but in the young bird their originally 
separate condition can still be traced (see Fig. 3). 
The fourth of these foot-bones is now reduced 
to a mere “ button ” of bone slung by ligaments 
on to the “ cannon ” bone. The leg of the old 
giant reptiles known as the Dinosaurs corre¬ 
sponds marvellously well with that of a modern 
bird, only in the reptiles the foot-bones had not 
become welded to form a cannon bone. So much, 
then, for the evidence from the skeleton, for the 
present. The arrangement of the blood-vessels, 
the structure of the eye, ear, organs of smell, 
and brain, are all on the reptile plan, and so, 
also, are the organs of reproduction. Similarly 
the microscopic structure of the growing 
feather in the embryo, or unhatched chick, 
shows that it is really an extremely elaborate 
reptile scale, and is formed on a plan quite dif¬ 
ferent from the hairs of mammals. 
EVIDENCE OF THE ROCKS 
But, it may be urged, it is all very well and 
it may be quite true to say that because the 
reptiles and the birds have so much in common, 
therefore they must be related. But why 
should we assume this? Well, one of two 
courses are open to us. Either we must believe 
that birds were, as used to be held, specially 
created, or that they have inherited these com¬ 
mon characters from a common ancestor, which 
must have been some sort of a reptile. And in 
support of the reasonableness of this latter view 
we may appeal to the evidence which the rocks 
have preserved for us in the shape of the fossil 
remains of ancestral birds. In these we have 
still further and more striking proof of the 
descent of birds from reptiles. 
The earliest fossil bird yet discovered is that 
known as the Archaeopteryx, and this differed 
from all other birds in one or two very impor¬ 
tant particulars. In each case they serve to 
bridge the gap between the reptiles and the birds, 
though it must be admitted many other links 
are desirable to make the chain absolutely com¬ 
plete. In the first place, instead of the horny 
sheaths which cover the beak of living birds, 
we find the jaws were provided with teeth, set 
in sockets like those of the crocodile, while in 
the second, the tail was of great length, being 
made up of a long row of bones, as in the tail 
of reptiles. Each bone supported a pair of 
feathers, as may be seen in our illustration, so 
that in this respect it was neither like that of the 
reptile nor of the typical bird. In the latter, 
the tail is apparently fashioned after a very 
different manner, but in examining this it must 
be remembered that what is commonly called 
the “ tail ” is really only the outward and visible 
sign of this appendage, for the feathers alone 
no more make the tail than do the hairs the tail 
of a dog. 
When we come, then, to examine the arrange¬ 
ment of the tail-feathers, we find that they are 
set fan-wise about a plate of bone, the last of a 
series of the eight separate tail bones which 
form the termination of the backbone. And if 
we examine this bony plate in the embryo of, 
say, a duck, we shall find that it is really made 
up of six or seven separate vertebrae, which have 
become, as it were, telescoped. Now each of 
these represents one of the feather-bearing ver¬ 
tebrae in the tail of the Archaeopteryx, but by 
this process of “ telescoping,” this shrinking in 
