Confining our attention to the large vertebrate classes, the testimony of the rocks proves, as we have said, that fishes appeared first in what are called the Silurian and Devonian epochs, where they developed into a rich and varied array of types unequaled in modern times.  At that period, they were the highest existing animals—­the “lords of creation,” as it were.  To change the figure, their branch constituted the top of the animal tree of the time, but as other branches grew upwards to bear their twigs and leaves, as the counterparts of species, the species of the branch of fishes decreased in number and variety, as do the leaves of a lower part of a tree when higher limbs grow to overshadow them.

Following the fishes, the amphibia arose during the coal age or Carboniferous, usurping the proud position of the lower vertebrate class.  The reptiles then appeared and gained ascendancy over the amphibia, to 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.  Palaeontology 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 palaeontological 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 zooelogical science as classics.  But even among the invertebrates significant cases may be found.  For example, in one restricted locality in Germany the shells of snails belonging to the genus