184 THE STOKY OP THE EARTH AND MAN. 



to the two lower classes of the vertebrates. The oldest 

 animal known to us is not only a creature of the 

 simplest structure, but also a representative of that 

 great and on the whole low type of animal life, in which 

 the parts are arranged around a central axis, and not 

 on that plan of bilateral symmetry which constitutes 

 one great leading distinction of the higher animals. 

 With the Cambrian, bilateral animals abound and be> 

 long to two very distinct lines of progress the one, 

 the Mollusks, showing the nutritive organs more fully 

 developed the other, the Articulates, having the 

 organs of sense and of locomotion more fully organized. 

 These three great types shared the world among them 

 throughout the earlier Palaeozoic time, and only in its 

 later ages began to be dominated by the higher types 

 of fishes and reptiles. In so far as we know, ifc re- 

 mained for the Mesozoic to introduce the birds and 

 mammals. In plant life the changes were less marked, 

 though here also there is progress land plants appear 

 to begin, not with the lowest fornje, but with the highest 

 types of the lower of the two great series into which 

 the vegetable kingdom is divided. From this they 

 rapidly rise to a full development of the lowest type of 

 the flowering plants, the pines and their allies, and 

 there the progress ceases ; for the known representatives 

 of the higher plants are extremely few and apparently 

 of little importance. 



Fifthly, in general the history tells of a continued 

 series of alternate victories and defeats of the species 

 that had their birth on the land and in the shallow 



