594 The Animal Kingdom 



their modern descendants, the kmgfish, it is probable that they also 

 had good lungs. 



From this meager evidence, some reconstruction of the probable 

 course of evolution of land-dwelling vertebrates can be made. It would 

 appear that sometime during the Devonian, mvich of the land was 

 swampy inasmuch as the waters of the ocean had receded. This condi- 

 tion left many water holes choked with vegetation and decaying organic 

 matter. Fish living in these areas would suffer from the depleted oxygen 

 supply, and only those forms capable of wriggling from pond to pond 

 and breathing some atmospheric oxygen could survive. Many species 



CHONDRICHTHYESV ^^-"""^ 

 (SHARK-UKE FI5HL5) ^ 



PLACODtRMI 

 _ (ARCHAIC ARMORED FlSHE-S) 



AG NAT HA 

 (JAWLESS FISHES) 



Fig. 201. — A simplified family tree to show the relationships of the vertebrate 

 classes. (From Romer: Man and the Vertebrates, The Univeristy of Chicago Press, 

 Copyright, 1941, The University of Chicago.) 



