Animals Before Ma 



11 



never again did tliey reacli their former im- 

 portance. This curious fluctuation in numbers 

 during the early part of their career is some- 

 what difficult to account for, but it may be that at 

 first, being for the time the leaders in life's race 

 and having no enemies, the sharks multiplied 

 until they actually became a check on one an- 

 other. Then the overspecialized forms, and 

 those that, on the other hand, failed to respond 

 to changing conditions, dropped out, leaving 

 those best fitted to survive. These in turn 

 gradually increased until Miocene times, when 

 they again suffered a reverse, this time very 

 likely through change of climate and a general 

 cooling of the oceans to the north and south of 

 the tropics. Be this as it may, while sharks at 

 the present day are found in the cold depths of 

 the Arctic Ocean as well as in tropical seas, 

 their headquarters are in the warmer portions 

 of the world. The empire of the sharks was 

 not one of long duration, for the day of their 

 supremacy saw the rise of air-breathing am- 

 phibians that were ere long to stand first 

 among living things, only to be deposed in 



110 



