become in the Mesozoic age the highest and most varied of the existing
vertebrates. At that time there were the great land dinosaurs with a
length of 80 feet, like _Brontosaurus_; aquatic forms like _Ichthyosaurus_
and _Plesiosaurus_, whose mode of evolution from terrestrial to swimming
habits was like that of seals and penguins of far later eras. Flying
reptiles also evolved, to set an example for the bats of the mammalian
class, for both kinds of flying organisms converted their anterior limbs
into wings, although in different ways.

During the Triassic and Jurassic periods of the Mesozoic age, the first
birds and mammals appeared to follow out their diverging and independent
lines of descent. Palæontology makes it possible to trace the origin and
development of many of the different branches that grew out of the
mammalian limb from different places and at different times during the
Mesozoic and the following age, called the Cenozoic, or age of recent
animals. It is unnecessary, however, for us to review more of the details:
the main result is obvious; namely, that the appearance of the great
classes of vertebrates is in the order of comparative anatomy and
embryology. Not only, then, is the fact of evolution rendered trebly sure,
but the general order of events is thrice and independently demonstrated
to be one and the same. Surely we must see that no reasonable explanation
other than evolution can be given for these basic facts and principles.

Turning now to the second division of palæontological evidence, we come to
those groups where abundant materials make it possible to arrange the
animals of successive epochs in series that may be remarkably complete.
For the reasons specified, the backboned animals provide the richest
arrays of these series, and such histories as those of horses and
elephants have taken their places in zoölogical science as classics. But
even among the invertebrates significant cases may be found. For example,