* * * * *

In the last place, we must give more than a passing consideration to some
of the extinct types of animals that occupy the position of "links"
between groups now widely separated by their divergence in evolution from
the same ancestors. Perhaps the most famous example is _Archæopteryx_
found in a series of slates in Germany. This animal is at once a
feathered, flying reptile, and a primitive bird with countless reptilian
structures. Its short head possesses lizard-like jaws, all of which bear
teeth; its wings comprise five clawed digits; its tail is composed of a
long series of joints or vertebræ, bearing large feathers in pairs; its
breastbone is flat and like a plate, thus resembling that of reptiles and
differing markedly from the great keeled breastbone of modern flying
birds, whose large muscles have necessitated the development of the keel
for purposes of firm attachment. In brief, this animal was close to the
point where reptiles and birds parted company in evolution, and although
it was a primitive bird, it is in a true sense a "missing link" between
reptiles and the group of modern birds. Other fossil forms like
_Hesperornis_ and _Ichthyornis_, whose remains occur in the strata of a
later date, fill in the gap between _Archæopteryx_ and the birds at the
present time, for among other things they possess teeth which indicate
their origin from forms like _Archæopteryx_, while in other respects they
are far nearer the birds of later epochs. That these links are not unique
is proved by numerous other examples known to science, such as those which
connect amphibia and reptiles, ancient reptiles and primitive mammals, as
well as those which come between the different orders of certain
vertebrate classes.

In summarizing the foregoing facts, and the larger bodies of evidence that
they exemplify, we learn how surely the testimony of the rocks establishes