thers, fan-like, to form the
so-called tail (Fig. 72). The _Archaeopteryx'_ tail is
_vertebrated_, the typical bird's _non-vertebrated_. This
shortening up of the tail did not take place at once, but
gradually. The Cretaceous birds, intermediate in time, had tails
intermediate in structure. The _Hesperornis_ of Marsh had twelve
joints. At first--in Jurassic strata--the tail is fully a half of
the whole vertebral column. It then gradually 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 development 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 still
passes through the stages of the phylogeny. The same is true of all
tailless animals.

[Illustration: FIG. 71.--Tail of _Archaeopteryx_. A indicates origin
of simply-jointed tail.]

[Illustration: FIG. 72.--Tail of modern Bird. The numerals indicate
the foreshortened, enlarged, and consolidated joints; _f_, terminal
segment of the vertebral column; D, shafts of feathers.]

[Illustration: FIG. 73.--_Archaeopteryx macura_, restored, 1/2 nat.
size. (After Flower.) The section of the tail is copied from Owen,
nat. size.]

The extinct _Archaeopteryx_ 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 vertebrated tail, to the
Sauropsidian type--or the trunk from which birds and reptiles have
diverged.

We will next consider the palaeontological evidence which we now
possess of the evolution of mammalian limbs, with special reference to
the hoofed animals, where this line of evidence happens to be most
complete.

I may best begin by describing the bones as these occur in the sundry
branches of the mammalian type now living. As we shall presently see,
the modifications which the limbs have undergone in these sundry
branches chiefly consist in the suppression of some parts and the
exaggerated development of others. But, by comparing all