habitats. But, as in the case of the dinosaurs, the elephants 

 have become rather specialized to adjust to their environ- 

 ment, which thus limits them to rather small regions now as 

 the world's jungles are being cut down by man. It appears 

 unlikely that these great vegetarians will be able to survive 

 much longer in the present world. 



Morern fish are the descendants of animals originating 

 from new Silurian vertebrates in the Devonian seas approxi- 

 mately 400 million years ago. Some of the primitive sharks 

 attained a size of 20 feet. In the succeeding geological 

 periods the fishes were present in many different types in 

 all the seas, and although they were dominated by larger 

 carnivorous aquatic reptiles (ichthyosaurs in the Triassic- 

 Cretaceous, and mososaurs in the Cretaceous periods), 

 they were able on the whole to escape predation. This was 

 due to their great agility in the water aided by their "lateral- 

 line" sensory system which detects disturbances in their 

 surroundings, and to their great breeding abilities. During 

 the Cretaceous period new types of fish closely related 

 to the present-day porbeagle sharks, rays, and herrings 

 evolved, and with the extinction of marine reptiles by the 

 time of the Eocene most fish species resembled the types 

 existing today. However further varieties of fish appeared 

 in the Miocene, including very large sharks over 60 feet 

 long. The latter died out in the Pliocene, while the other fish 

 species changed very little on to the present time. In the 

 fish we see therefore a line of animals, lasting at least 400 

 million years, in which a fairly standard pattern has resulted 

 in practically perfect adaptation to the liquid environment. 

 Since the overall conditions in the seas do not change 

 appreciably even during Ice Ages there is every likelihood 

 that fish will continue to populate the waters for a long 

 time yet. But their severe limitation of being confined to 



