THE ORIGIN OF THE ANIMAL KINGDOM. 83 



soon as the descendants of this simple type become 

 modified in any direction, the future development is 

 immediately limited to the type thus produced. 

 When some of the early animals modified their 

 digestive organs so that there were two openings to 

 the digestive tract instead of one, from that time 

 their descendants were required to conform to this 

 type. When some of them developed an elongated 

 segmented body, it was no longer possible for them 

 to return to the Gastraea stage, and start in a new 

 direction. Advance in structure comes from as- 

 sumption of different functions on the parts of 

 organs originally alike, and as the parts of the body 

 acquire a wider and more varied development, the 

 possibility of further differentiation is more and 

 more restricted. The further the development of 

 animals progresses, therefore, the less will be the 

 modifications of structure that take place, and the 

 more must development be confined to the elabora- 

 tion of details. This fact we find constantly illus- 

 trated throughout the history of animals, and it will 

 be frequently referred to hereafter. 



We can thus understand why the sub-kingdoms 

 which arose early in the life of animals could con- 

 tinue to advance and elaborate each its own type by 

 the production of many minor branches, but why 

 none of them could give rise to new types which 

 would present as great differences as the sub-king- 

 doms first appearing. If the Gastrsea ancestor had 

 remained in existence to serve in future ages as the 

 starting-point of new lines of differentiation, then 

 the production of great divisions would seemingly 



