THE VOYAGE OF EVOLUTION 109 



forms of life it must have air. As it cannot get this from the 

 water, it builds a little tube from its mouth to the door of its 

 chamber. Thus it actually breathes. Dry balls of mud contain- 

 ing such fish have often been brought to Europe. When soaked 

 in water the fish emerge as lively as ever. 



Such fishes make their first appearance in the early geological 

 era known as the Devonian. How far back this was may be 

 judged by a reference to Figure 21 which shows the names of the 

 geological periods, the conditions of climate, and some of the 

 chief crises in the evolution of life. In the Devonian Period the 

 former moist climate gave place to aridity. Throughout large 

 parts of the earth vast deserts prevailed, as is evident from great 

 deposits of red sandstone. In the early stages of this desiccation 

 a great advantage was possessed by those fresh-water fish which 

 were able to act like the modern lung fish. As the climate became 

 drier, forms which were highly developed in this direction were 

 the only ones that were able to survive in many regions. More- 

 over, those fish which could crawl to new pools also had a great 

 advantage, for when a stream became low they could move down 

 its bed from their own diminished pool to a larger one. Today 

 in Ceylon the climbing perch is able to use its fins in this way. 

 Thus the dryness of the Devonian Period caused the evolution of 

 legs as well as lungs, and there arose the class of animals known 

 as amphibians. 



For millions of years amphibians were the highest type of life. 

 Little by little their lungs and legs became better developed until 

 the creatures seemed to belong wholly to the land. Nevertheless 

 they still returned to the water to lay their eggs. Their young 

 passed through a fishlike stage such as we see not only in tadpoles 

 but in the embryos of man and the higher animals. They had 

 not yet taken the final step to the top of the great terrace where 

 the land animals dwell. Finally, however, in the latter part of 

 the period known as the Mississippian there came another time 

 of climatic stress. Aridity once more became so pronounced 



