GENERAL SUCCESSION OF ORGANIC TYPES. 55 



means contended that the Ganoid fishes of the Upper Silurian 

 Rocks were in any way degraded members of their order, or 

 inferior in point of organisation to the Ganoids of the present 

 day. On the contrary, there is reason to think that many 

 types early presented a development more varied than that 

 exhibited by their successors. It is simply contended that, on 

 the whole, there has been a zoological progression as we ascend 

 from the Cambrian period to the present day. It is also to 

 be remembered, that though the commencement of the Inver- 

 tebrate sub-kingdoms may be unknown to us, a similar pro- 

 gression can be in many cases shown as regards the orders and 

 classes of these, even more completely than in the case of the 

 Vertebrate sub-kingdom. 



