2 THE HORSE 



remains of animals in the respective strata affords a 

 reliable guide to the sequence of their evolution, and 

 shows the different stages they have passed through to 

 attain their modern form ; while their total absence in 

 the early geologic formations is conclusive proof that they 

 must have evolved from the primitive forms, which then 

 alone peopled the earth. 



To commence at the beginning, there was first the 

 Pre-Cambrian Age, that of the Torridonian Sandstone 

 and various igneous rocks. No undoubted fossils in this 

 formation have yet been found, but life most probably 

 existed, as we find all the chief genera of invertebrates 

 in the succeeding Cambrian period. Following in 

 sequence came the Primary, or Palteozoic epoch, 

 commencing with the Cambrian formation, when 

 Trilobites were in existence. Later was included the 

 formation of the Silurian, when there were plant-like 

 animals, and small fish ; the Devonian, when Ganoid 

 fishes and true sharks first appeared ; the Carboniferous, 

 when (as the coal measures show) there were great 

 forests, and plant-life was on a profuse scale ; and lastly, 

 the Permian — a formation of the limestone, slate, and 

 sandstone, when reptiles first began to crawl. 



Following this epoch came the Secondary, or Mesozoic 

 Age. This included the Triassic; the Jurassic, (the 

 flourishing period of Ammonites and reptiles, such as the 

 Icthyosaurus, Deinosaurus, and Plesiosaurus, and when 

 the first bird made its appearance) ; and lastly, the 

 Cretaceous, or Chalk Age, when the remains have been 

 found in America of birds possessing teeth ! Whether 

 mammals evolved from such birds or from reptiles yet 

 remains to be proved. The evolution of fish into land- 

 animals may possibly be traced through the curious cat- 

 fish of Africa, and the climbing perch of India, both of 

 which can live out of water for a considerable time. In 

 the former this power is due to vascular structures above 

 the gills, enabling them to breathe atmospheric air. One 

 form is almost amphibious, and when the marshes are 

 dried up can spend the dry season in burrows, which it 



