86 THE LIVING WORLD. 



to any later point of origin, but probably most, if 

 not all, of the sub-kingdoms can be independently 

 traced to this one point, indicating that this was the 

 starting-point of all the animal kingdoms. It is 

 doubtful whether even two of the great types exist- 

 ing to-day have had a common point of origin later 

 than the Gastraea. This has been seen to be due to 

 the fact that the great types are separated from each 

 other by differences in fundamental plan of structure, 

 and such fundamental differences could only have 

 arisen from an undifferentiated form which had as 

 yet developed no distinct type of its own. 



