98 THE LIVING WORLD. 



except the vertebrates, since practically all of them 

 had developed before the fossil record began. It is 

 true that in the subsequent ages most of the other 

 sub-kingdoms have very much expanded in variety 

 of forms, and have in general seized upon larger and 

 larger fields in nature. But in most of them the 

 real advance has been slight, and in many there has 

 been actually no advance, but only production of 

 new genera and species. In the vertebrates alone 

 has all of the development taken place during the 

 period of which we have fossil record, and here alone 

 can we read a definite history of progression, since 

 here alone can we trace anything like a continuous 

 history. 



With the opening of the Silurian (2), therefore, 

 the animal kingdom bursts upon us in a compara- 

 tively high state of development. From this time 

 on there is a constant widening of types, a constant 

 succession and disappearance of old forms and the 

 appearance of new ones. 



As already seen, the Gastraea was the last point in 

 the history of life shared in common by all multicel- 

 lular animals. From this point there was a parting 

 of the ways, and it is therefore no longer possible to 

 follow the history of the animal kingdom as a whole. 

 It will be necessary to take up the different branches 

 and follow them separately. Of course, the further 

 we trace them the greater and more complex will 

 become the branching, but it is not our intention to 

 trace this history in very great detail. It will be neces- 

 sary, however, here to take into brief consideration 

 the history of the various sub-kingdoms of animals 



