6o FROM FISH TO PHILOSOPBER 



and dry seasons. The red sandstone that gave the system 

 its earlier name is an oxidized iron-containing rock that 

 must have been formed from mud periodically exposed 

 to air; but the thick layers of greenish-gray sandstone, 

 siltstone, and gray shale interlarded with the red sand- 

 stone must have been formed in more permanent shallow 

 waters, or in perpetually moist mud lakes. Whereas in 

 the Lower Devonian the continental waters had been 

 displaced by mountains drained by torrential rivers, by 

 the Middle Devonian, when these mountains had been 

 eroded to a considerable extent, large areas of the low- 

 lands had become deserts of wind-blown sand and all 

 habitable portions of the continents were subjected to 

 extremes of climate. As annual drought succeeded an- 

 nual flood, the evanescent lakes gave way to stagnant 

 pools and hard mud flats in a cycle such as is commonly 

 seen today in Australia, Central Africa, Eurasia, and the 

 western part of North America; and most of the Devo- 

 nian, fifty million years in length, presents a geologic 

 record of, in alternation, too much and too Httle rain. 

 The vertebrates, now represented by fishes of an ad- 

 vanced type, had to choose between living in stagnant 

 pools and dry mud flats for half the year, or seeking 

 sanctuary in the stable waters of the sea. 



Two primitive groups of fishes chose the latter course. 

 One, the Arthrodira, became extinct at the end of the 

 Devonian and we cannot even speculate how they main- 

 tained their salt and water balance, or why they became 

 extinct. But the second group survived to establish the 

 marine cartilaginous fishes, the class Elasmobranchii 

 ( = Chondrichthyes), represented today by the sharks, 

 rays, skates, and chimaeroids. 



It is appropriate to emphasize here that there are two 

 great groups of fishes that differ from each other in many 

 notable respects: the cartilaginous fishes or the Elasmo- 

 branchii (elasmos = plate; branchia = gill), or, as they 

 are sometimes called, the Chondrichthyes {chondros=- 

 cartilage; ichthys = fish), which in respect to evolution 



