42 LAND LIFE BEGINS 



understand the evolution of life. The decreasing 

 lakes and rivers and shallow seas were overcrowded. 

 There was a fearful fight for oxygen and for food. 

 For those which ventured back into the deep seas 

 there were sharks thirty feet long, with teeth five 

 inches long, and other great fishes with jaws two feet 

 in width, waiting. There was, in a sense, a race 

 for the land — that is to say, an increased development 

 of adaptation to land life. Lungs for breathing air 

 were the chief things required ; and we find lungs (or 

 other air-breathing organs) developing on all sides. 

 The fish would develop its lungs out of the ''floating 

 bladder," or gas-bag, which it already had for swim- 

 ming purposes. In some fishes this bladder is double, 

 and is already connected with the gullet by tubes. 



Naturally, I can give here only a very superficial 

 account of what happened, but all the details of the 

 change to land life will be found worked out in larger 

 scientific works. For my purpose it is enough to 

 describe how, at the time when the primitive ferns 

 and mosses spread over the warm earth (in what a 

 geologist calls the "Devonian Period," because its 

 most characteristic rocks are those red sandstone 

 cliffs you may have seen in Devonshire), a great 

 swarm of uncouth lung-breathing fishes covered the 

 land. No doubt they "kept one leg in the water/' 

 so to say. They preferred to live in water as far as 

 they could. But there was a desperate struggle for 

 food in the crowded waters, while the land had as yet 



