SEPARA TION OF THE SUB-KINGDOMS. 1 59 



ones and finally into a common trunk. And studies 

 since Darwin have tended to show that the gas- 

 trula is the representative of this common trunk, 

 and that from it all of the great groups branched off 

 directly. It is one of the problems now before 

 embryologists to decide as to the number of the 

 branches which arose from this ancient ancestor, 

 whether indeed the meaning of the groups which 

 we recognize as sub-kingdoms, is not that each rep- 

 resents one of these branches from the gastrula, and 

 if so to determine how many sub-kingdoms we 

 should find. For instance, whether the Arthropoda 

 (insects, etc.) and Annelids (worms) should be regard- 

 ed as separate branches, or whether they arose in 

 common from the gastrula and soon separated from 

 each other. 



This early divergence of the great types from 

 each other is one of the important deductions of 

 recent embryology, since it enables us to meet one 

 of the problems we discoursed in the last chapter, 

 viz.: the simultaneous appearance of the types in 

 the Silurian. One extreme view derives seven or 

 eight branches directly from the ancient common 

 ancestor: I coelenterates, 2 polyzoa and brachio, 

 pods, 3 annelids, 4 arthropods, 5 mollusks, 6 echino- 

 derms, 7 vertebrates, and perhaps another to 

 include the heterogeneous group vermes. Now if 

 any such conclusion as this is right, we immediately 

 find an explanation for the Silurian fauna. All of 

 the types which have been found in the Silurian 

 rocks developed independently and simultaneously 

 from the ancient common ancestor; and it is no 



