84 THE LIVING WORLD. 



have been indefinitely continued. But such does 

 not appear to have been the case. In other words, 

 the evidence of nature seems to indicate that the 

 different sub-kingdoms did not develop from each 

 other, but all (?) diverged early in the history of the 

 world from a simple undifferentiated animal which 

 bore some resemblance to the hypothetical Gastraea. 

 This conclusion is one of no little significance, for it 

 indicates that the origin of the large types of ani- 

 mals was a matter much more rapid than their sub- 

 sequent elaboration. At the origin of the multi- 

 cellular animals, a comparatively short time probably 

 produced divergence in the type which gave rise to 

 the great sub-kingdoms, while the millions of years 

 that succeeded only sufficed to elaborate and differ- 

 entiate these types. This understanding gives us an 

 explanation of the interesting fact that no new 

 types have appeared within the geological periods 

 since the Silurian, and it assists very much in under- 

 standing what seems to be the sudden appearance 

 of life in the oldest fossiliferous rocks. 



Summary. 



In the study of the early history of life three 

 points of importance stand prominently forth. 



First : Far back at the beginning of life on the 

 globe, there was a period during which the unicellu- 

 lar organisms were the highest that existed. Al- 

 ready the plant life and the animal life had become 

 separate. We do not understand, however, that 

 this unicellular stage was the beginning of the his- 

 tory of life, for we are learning that there is a vast 



