COURSE OF DEVELOPMENT ON THE EARTH. 145 



existing types are always arising, and increasing 

 differentiation, and consequent improvement is 

 thus secured. 



If we survey the progress of Vertebrate 

 animals, we can trace four great periods. First, 

 there was a time when the earth seems to have 

 been almost covered with water, and when fish 

 of all sorts and sizes predominated. Most of 

 these were ganoid fishes, of a type which has 

 now very few existing representatives. When 

 the earth became more solid, an era of reptiles 

 succeeded. The greater part of these died out, 

 and an era of warm-blooded animals followed. 

 Then came the commencement of the Human 

 Period, and man is now more widely distributed 

 and dominant than any other animal or group, 

 recent or fossil. 



His present pre-eminence does not correspond 

 with that of any other species, however domi- 

 nant or widely distributed, but rather with that 

 of a whole Class, an analogy which seems to 

 have been overlooked in most discussions on 

 his position. 



In each era the highest animals which the 

 earth was then fitted to sustain have always 

 been dominant, while the inferior groups which 



