The Development of the Animal World 77 



Once more, if there is some obscurity about the first 

 development of the fish, there is ample evidence of the 

 action of evolution on the whole of its subsequent 

 career. The earliest fishes we find, in the early 

 Devonian, are of the simplest type — the shark and ray 

 type (Elasmobranchs) — though some of the sharks soon 

 attain the most formidable proportions, and have their 

 jaws lined with saw-like triangular teeth five or six 

 inches long. From these Elasmobranchs two great 

 groups are developed in the Devonian waters, the 

 Ganoids or scaly fishes and the Dipnoi or double 

 breathers (having both gills and lungs). We are now at 

 a great crisis in the development of animal life. Had 

 the vertebrates remained in the water, their intelligence 

 would not have advanced beyond that of the salmon. 

 But with the multiplication of gigantic fishes, and the 

 enormous development of teeth and armour (bony 

 plates), the struggle in the waters became intense, and 

 at the same time the land, as we saw, began to rise from 

 the deep. Large tracts of the sea were caught in the 

 rising continents and converted into the inland Devonian 

 lakes, where the red sandstone was laid down. The 

 struggle became fiercer in these shrinking lakes, and 

 adaptation to land life became a most valuable advan- 

 tage. It was then that the first fishes developed lungs 

 for breathing the air, and started that new colony of 

 land dwellers that was to culminate in man. Of the 

 fishes that remained in the water we must compress a 

 long story in a sentence, by saying that our sharks and 

 dog-fish still represent the early cartilaginous type ; but 

 the Ganoids gave way before the later bony fishes 

 (Teleosts), which appeared in the Jurassic, and branched 

 out into the numerous types with which we are familiar. 

 The Dipnoi claim closer attention. 



In three different parts of the world — Australia, South 



