THE FUTURE OF THE LIVING WORLD. 185 



one. The sub-kingdoms have not arisen independ- 

 ently from the Protozoa, but have all had a common 

 ancestor, the Gastnea, and this means that only once 

 has the unicellular form given rise to an important 

 line of multicellular descendants which perpetuated 

 itself. Though the Coelentera stand very near this 

 primitive Gastraea, there is no evidence that the 

 sub-kingdom has the power of further production of 

 new types ; but, on the contrary, everything tends 

 to show that, whatsoever differentiation of this 

 simple type ever did take place to give rise to the 

 sub-kingdoms, occurred before the Silurian. Since 

 paleontology shows that no great types have arisen 

 since the Silurian, it is plain that all of the expansion 

 of the simple unicellular form must have taken place 

 before the Silurian. And coming through the later 

 ages, we find that the evidence is the same in its 

 tenor. The conclusion everywhere seems to be that 

 when a generalized form has given rise to one or two 

 lines of development, it either disappears or loses its 

 power to originate new forms. Every bit of evi- 

 dence which indicates a fundamental unity of the 

 animal kingdom testifies to the same. Without 

 questioning the theoretical possibility that any or 

 all of the existing unspecialized forms may in the 

 future develop, we must acknowledge that the 

 probability is against it. Nothing in history in- 

 dicates that these groups retain the power to ex- 

 pand, and there is, therefore, no reason for thinking 

 it a possibility in the future. Remembering what a 

 large number of groups we are learning to trace 

 back to the Silurian, remembering that development 



