Paleontolory. 173 
earliest of all were almost certainly like Fig. 70; then they be- 
came like Fig. 69; and, finally, only much later in geological 
history (Jurassic or Cretaceous), they became like Fig. 68. This 
order of change is still retained in the embryonic development 
of the last introduced and most specialized order of existing 
fishes. The family history is repeated in the individual history. 
Similar changes have taken place in the form and structure of 
birds’ tails. The earliest bird known—the Jurassic Archao- 
pteryx—had a long reptilian tail of twenty-one joints, each joint 
bearing a feather on each side, right and left (Fig. 71) : [see also 
Fig. 73]. In the typical modern bird, on the contrary, the tail- 
Joints are diminished in number, shortened up, and enlarged, 
and give out long feathers, fan-like, to form the so-caled tail 
(Fig. 72). The Archeopteryx' tail is vertebrated, the typical 
bird’s xon-vertetrated. This shortening up of the tail did not 
take place at once, but gradually. The Cretaceous birds, inter- 
mediate in time, had tails intermediate in structure. The Hes- 
perornis of Marsh hadtwelve joints. Atfirst—in Jurassic strata— 
the tail is fully a half of the whole vertebral column. It then grad- 
ually shortens up until it becomes the aborted organ of typical 
modern birds. Now, in embryonic development, the tail of the 
modern typical bird passes through all these stages. At first the 
tail is nearly one half the whole vertebral column ; then, as de- 
velopment goes on, while the rest of the body grows, the growth 
of the tail stops, and thus finally becomes the aborted organ we 
now find. The ontogeny sti!l passes through the stages of the 
phylogeny. The same is true of all tailless animals. 
The extinct Archeopteryx above alluded to presents 
_ throughout its whole organization a most interesting 
assemblage of “generalized characters.” For example, 
its teeth, and its still unreduced digits of the wings 
(which, like those of the feet, are covered with scales), 
refer us, with almost as much force as does the verte- 
brated tail, to the Sauropsidian type—or the trunk 
from which birds and reptiles have diverged. 
We will next consider the paleontological evidence 
