CHAP, i.] THE THREE PHASES OF LIFE ON THE EARTH. 7 



ceous nocks of America by the Odontornithes or birds 

 with teeth in their beaks. 1 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 Keptilia 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 con- 

 sisted principally of conifera, araucarice, zamiw, 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 universal. 



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 



1 Marsh, American Journal of Science and Arts, x. Nov. 1875. 



