ened, for commercial
purposes, in the second half of the nineteenth century, critics of
evolution--if there still were any in the world of science--might be
repeating to-day that the transition from the reptile to the bird was
unthinkable in theory and unproven in fact.

The features of the Archaeopteryx ("primitive bird") have been described
so often, and such excellent pictorial restorations of its appearance
may now be seen, that we may deal with it briefly. We have in it a most
instructive combination of the characters of the bird and the reptile.
The feathers alone, the imprint of which is excellently preserved in
the fine limestone, would indicate its bird nature, but other anatomical
distinctions are clearly seen in it. "There is," says Dr. Woodward, "a
typical bird's 'merrythought' between the wings, and the hind leg
is exactly that of a perching bird." In other words, it has the
shoulder-girdle and four-toed foot, as well as the feathers, of a bird.
On the other hand, it has a long tail (instead of a terminal tuft of
feathers as in the bird) consisting of twenty-one vertebrae, with
the feathers springing in pairs from either side; it has biconcave
vertebrae, like the fishes, amphibia, and reptiles; it has teeth in its
jaws; and it has three complete fingers, free and clawed, on its front
limbs.

As in the living Peripatus, therefore, we have here a very valuable
connecting link between two very different types of organisms. It is
clear that one of the smaller reptiles--the Archaeopteryx is between a
pigeon and a crow in size--of the Triassic period was the ancestor of
the birds. Its most conspicuous distinction was that it developed a
coat of feathers. A more important difference between the bird and the
reptile is that the heart of the bird is completely divided into four
chambers, but, as we saw, this probably occurred also in the other
flying reptiles. It may be said to be almost a condition of the greater
energy of a flying animal. When the heart has four complete chambers,
the carbonised blood from the tissues of the body can be conveyed direct
to the lungs for purification, and the aerated blood taken direct to the
tissues, without any mingling of the two. In the mud-fish and amphibian,
we saw, the heart has two chambers (auricles) above, but one (ventricle)
below, in which the pure and impure blood mingle. In the reptiles a
partition begins to form in the lower chamber. In the turtle it is
so nearly co