Page:Early Man in Britain and His Place in the Tertiary Period.djvu/35

] rocks of America by the Odontornithes or birds with teeth in their beaks. The remarkable combination of characters in these two widely-different forms renders it probable that the class Aves was not sharply defined from the class Reptilia in the Secondary period. The only mammalia which have been discovered are small marsupials, the largest of which was not larger than a kangaroo-rat. The forests then covering Europe consisted principally of coniferæ, araucariæ, zamiæ, and cycads, and trees with deciduous leaves do not appear until the Cretaceous or concluding phase of the period. In the third or Tertiary period, the higher placental mammals first appear, taking the place of the reptiles of the Secondary in their mastery of land and sea, and flying in the air as bats. The true birds also have left the reptilian characters far behind, and in the vegetable kingdom the angiosperms, both evergreen and with deciduous leaves, increase and multiply, until they assume their present important place in the forests of the world.

These three phases of life may be traced over the whole earth, and their succession is invariable, from which it may be inferred that they are due to causes acting universally, and not sporadically in one or more centres. They prove that the earth as a whole has passed through a series of biological changes, analogous to those which are to be seen in the animal world in the passage from birth to old age. They may be accounted for on the theory of evolution of Herbert Spencer and