LIFE. 501 



approach Reptiles and Birds in their semi-oviparous character ; they 

 are like the former also in the sacrum consisting of but two verte- 

 bras combined ; and among the lower species of later time the 

 birds are represented by the Duck-bill (Ornithorhynchus) and Rep- 

 tiles by the Echidnus. Along with these inferior features there were 

 the dominant characteristics of the true Mammal. 



A subordinate example of the comprehensive type is seen in the 

 Rhynchosaur of the Trias, which combined the characteristics of 

 the Saurian with the bill, and partly the skull, of a Turtle. Whe- 

 ther it had paddles or not is undecided. 



All the examples here mentioned, it will be observed, made 

 their appearance in the Trias. 



The Pterodactyls combine with the Saurian characteristics peculiarities of 

 birds ; but they arc not satisfactory examples of a comprehensive type, any 

 more than the Bats among Mammals : they are adaptations of the Reptilian 

 type to the air. 



Earliest Mammals. — On p. 39G several examples are mentioned in 

 which the early species of a group were partly from the lower of its 

 two grand subdivisions, and partly from the inferior grades of the 

 higher. If, in a similar manner, Mammals are divided into Marsu- 

 pials (the lower division) and Non-marsupials (the higher), the ear- 

 liest kinds embrace species of the former, along with Insectivores, 

 a tribe in the inferior or Microsthenic division (p. 423) of the 

 latter. 



The species of the higher group are not necessarily from its lowest sub- 

 division. Below the Insectivores there is the Sloth tribe, or that of 

 Edentates ; and this low group is not known to occur before the 

 Cenozoic (Tertiary). In the same manner the Reptilian associates 

 of the early Amphibians were Lacertians and Enaliosaurs, and not 

 Snakes of the inferior subdivision of True Reptiles. The earliest- 

 known Snakes are found in the Tertiary. 



In the limited Fauna and Flora of early time the species of the 

 higher group are such as blend harmoniously with those of the lower. This 

 principle is illustrated on p. 396. In the case of Mammals, the 

 Marsupial Insectivores harmonize better with the Non-marsupial 

 Insectivores than they would with Sloths, and the Labyrintho- 

 donts better with Lacertians than with Snakes. Again, the air- 

 breathing Vertebrates of the Mesozoic, made up of oviparous Rep 7 

 tiles, semi-oviparous Mammals, and viviparous Mammals, are an 

 harmonious assemblage. The fauna of an era is not well appre- 

 ciated unless considered apart from those of other periods. An 

 attempt to classify the living species as they would have appeared 



